Undercover operation (5)
I believe the answer is:
sting
'undercover operation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sting that I've seen before include "Entrapment operation; sharp pain" , "Bee's weapon" , "English musician, b. 1951 -- a sudden pain" , "fleece?" , "Con: leader singer of Police" .)