Police spy or informer (4)
I believe the answer is:
nark
I believe this clue is a double definition.
'police spy' is the first definition.
(I've seen this before)
'informer' is the second definition.
(I know that nark is a type of informer)
(Other definitions for nark that I've seen before include "a nuisance down under" , "Bother" , "cause irritation" , "Annoy; informer (sl.)" , "expert" .)