Infiltrator (4)
I believe the answer is:
mole
'infiltrator' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mole that I've seen before include "Spy - one that burrows" , "A spy within an organisation" , "Small congenital skin spot" , "Subterranean mammal" , "spy a > breakwater" .)