Sleeper agent (4)
I believe the answer is:
mole
'sleeper agent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mole that I've seen before include "Skin spot; breakwater" , "Underground worker!" , "Burrowing mammal with velvety fur" , "spy a > breakwater" , ""Gentleman in black velvet''" .)