Pusher's chaser (4)
I believe the answer is:
narc
'pusher's chaser' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for narc that I've seen before include "Agent dealing with Charlie" , "Drugs agent (US)" , "member of drug squad" , "One on drugs squad" , "Drugs officer" .)