Leader of gang completely fleeced victim (4)
I believe the answer is:
gull
'victim' is the definition.
(gull is a kind of victim)
I don't understand the remainder of the clue.
Can you help me to learn more?
(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" .)