Sleeper spy (4)

I believe the answer is:

mole

(Other definitions for mole that I've seen before include "Pier" , "Skin spot; breakwater" , "Small burrowing animal" , "sauce" , "Burrowing mammal with velvety fur" .)

I've seen this clue in The Wall Street Journal.

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.