Annoy an informer (4)
I believe the answer is:
nark
This clue is a double definition.
'annoy' is the first definition.
(I know that annoy can be written as nark)
'an informer' is the second definition.
(I know that nark is a type of informer)
(Other definitions for nark that I've seen before include "cause irritation" , "Get to" , "upset" , "Stool pigeon" , "Bug" .)