Informer or spy (4)
I believe the answer is:
This clue is a double definition.
'informer' is the first definition.
(I know that nark is a type of informer)
'spy' is the second definition.
(I've seen this before)
(Other definitions for nark that I've seen before include "Informer - irritate" , "Get to" , "Police spy" , "expert" , "Keep an eye" .)