Police informer (4)
'police informer' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stool pigeon that I've seen before include "Police informer" , "Grass" .)
'police informer' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nark that I've seen before include "source of irritation" , "Cooper's .... was old police informer" , "Cause minor annoyance" , "Keep an eye" , "Habitual grumbler" .)