Drug agent managed to get back cocaine (4)
I believe the answer is:
narc
'drug agent' is the definition.
(narcotics officer)
'managed to get back cocaine' is the wordplay.
'managed' becomes 'ran' (both can mean organised or was in charge of).
'to get back' shows that the letters should be reversed in order (I've seen 'getting back' mean this).
'cocaine' becomes 'C' (drugs abbreviation).
'ran' backwards is 'nar'.
'nar'+'c'='NARC'
(Other definitions for narc that I've seen before include "Agent dealing with Charlie" , "Drugs officer" , "Drugs agent (US)" , "member of drug squad" , "One on drugs squad" .)