Leader of gang completely fleeced victim (4)

I believe the answer is:

gull

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'victim' is the definition.
(gull is a kind of victim)

I don't understand the remainder of the clue.

(Other definitions for gull that I've seen before include "A bird; to deceive" , "Dupe - sea bird" , "Mostly white aquatic bird" , "Seabird with a raucous call" , "Take in" .)

I've seen this clue in The Independent.

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