Taxi Driver 👀

Ones that immediately come to mind -

1. Taxi Driver - Taxi company as a cover and a tool for a team of vigilantes
2. The Spies Who Loved Me - Book publishing company as a front for an espionage arm of Interpol!
Healer and City Hunter also had the MLs taking on everyday jobs as a cover but I guess they wouldn't fall in the legitimate business vs something else category.

Sell Your Haunted House - they were real estate agents but also ghost hunters! And also the story in the Uncanny Counter where the group run a restaurant business but are also demon hunters. Hahaha, had to do some research on my previously watched dramas list!

Mystic Pop-Up Bar.

Blade Man, perhaps? Not that I'd recommend it though, in spite of my love of Lee Dong Wook.

Soooooo if we want to be a bit more liberal with our take of what is considered a business front and subsequent clandestine activities….

Remarriage & Desires
Our protagonist becomes a tutor to a son of some CEO to stop her nemesis from seducing said CEO (but really I think she was in love with her nemesis and only married the CEO because her nemesis wanted to)

Cruel/ Heartless City

Our Female Lead (after her sister’s unfortunate death) works with the police and becomes an escort to gain intel on some drug lord baddie ( forgive me for the lack of specific details since this drama aired waaaay back in 2013, but actor Jung Kyoung-ho was seriously HOT no?)

Extreme Job - A group of cops hide their undercover activities by opening a chicken restaurant. (This is a movie, not a drama, but it's really fun. )

The Uncanny Counter- Noodle restaurant employees who, you know, also hunt supernatural beings with their supernatural abilities. Everyone has to have a side hustle these days :p

1. Hwayugi A Korean Odyssey - There's an entertainment company, but everyone from the CEO Mawang (Bull Demon King) to the employees and artists are all some kind of immortal demons or monsters.
2.Vampire Prosecutor- the titular Vampire Prosecutor himself might fit the description a bit, but there's also a bar that after hours serves blood not just fancy drinks.

Big fan of the concept, but don't seem to have many reccs for it. To be honest, overall, my recommendation list is not big anyway, as I'm pretty picky about what I genuinely can and want to recommend to people, especially if it's not a direct one on one recommendation.

Semantically, I don't think I have anything that fits the exact criteria- if it needs both legitimate business cover and clandestine operations behind the front (and it's a good recommendation).

For just the clandestine, and a cover of sorts at least for half the show then obviously Healer, hands down.

Mystic Pop Up Bar and Sell Your Haunted House are both normal business fronts as covers for supernatural businesses, but neither are clandestine in the illicit connotation.

Then we get into stuff that pushes the criteria even farther, but I would still recommend--
Extracurricular - High School Kid runs a security company for illegal prosititutes. If being a high schooler counts as a legitimate business front that is haha.
My Fellow Citizens - Con Man runs for National Assembly.
Tale of the Nine Tailed as a couple of Gumiho's running business fronts (one is a vet) from what I remember, but they also just hang out normally sometimes too lmao...

38 Task Force as a team of cops and conartists who work together (not necessarily legally) to catch tax evaders sort of counts?
Plenty of minor undercover roles in any thriller or cop show, come to mind, but they're not explicitly "business" fronts -E.g. Military Prosecutor has the FL pose as a vigilante on the side of her Military job, but it's minor and basically disappears in the back half- and we get into a too broad a criteria if we do that I think.

(I was gonna out My Love From the Stars here too but I can't remember what Do Min Joon's day job is...)

Aaand then stuff that is a bit harder for me to recc--
Kill It- an Assassin has a day job as a Vet.
I did like Kill It but it's not very good lol. Fits the criteria perfectly otherwise though 😂

And a bit harder again--
Spies Who Loved Me is fun but the story is terrible and also pointless and is only worthwhile for the amazing Female Lead, Yoo Inna as Areum.
The business front part of the Interpol agents is minor, but the FL runs a dressmaking company whilst conducting her investigations along side the spies, so. That's pretty cool. She also gets to stab some people with her dressmaking scissors. (Yoo Inna Spy Cinematic Universe Part 3 when)

My Name, Witch's Diner, From Now On: Showtime, Vincenzo, Taxi Driver, Snowdrop, Undercover, River Where the Moon Rises (Herbalist is a Silla Spy lmao, oh what that concept could've been, alas), all count but I do not recommend any of them.

The enjoyable romp that was My Secret Terrius had bag shop PPL as a cover for an NIS operation (and the bad guys were using luxury hand bags for their evil schemes as well).
They even had a secret base behind the shop and the ongoing gag of the oblivious female lead insisting on being the best at selling handbags and drawing unwanted customers to the store.

Not a demon or ghost hunter or vigilante in sight but I found it moving that in “Just between lovers”, Jeong Sookhee the loan shark, also operated an underground health clinic for poor people. Na Moonhee was so great in that role and I loved her rapport with Junho’s Kangdoo.