An informer to annoy (4)
I believe the answer is:
nark
This clue is a double definition.
'an informer' is the first definition.
(nark is a kind of informer)
'annoy' is the second definition.
('nark' can be a synonym of 'annoy')
'to' is the link.
(Other definitions for nark that I've seen before include "He gives the game away" , "Grass; irritate" , "Bug" , "upset" , "Annoy; informer (sl.)" .)