Certain undercover agent (4)

I believe the answer is:

mole

(Other definitions for mole that I've seen before include "beauty spot" , "Subterranean mammal" , "Small burrowing animal" , "Heap of soil (a 6 mountain?)" , "Tunneller" .)

I've seen this clue in the Newsday.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.