Drug agent - it's very fishy looking back (4)

I believe the answer is:

narc

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'drug agent' is the definition.
(narcotics officer)

I don't understand how the remainder of the clue works.

(Other definitions for narc that I've seen before include "Drugs agent (US)" , "member of drug squad" , "Drugs officer" , "Agent dealing with Charlie" , "One on drugs squad" .)

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