British stoolie (4)
I believe the answer is:
nark
'british stoolie' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nark that I've seen before include "Bug" , "Annoy; police informer" , "Cause minor annoyance" , "Habitual grumbler" , "Annoy (colloq.); informer" .)