Top individual, leader of gang (6)
I believe the answer is:
capone
'leader of gang' is the definition.
(gangster Al Capone)
'top individual' is the wordplay.
'top' becomes 'cap' (both can mean to beat or outdo).
'individual' becomes 'one' (an individual is one person).
'cap'+'one'='CAPONE'
(Other definitions for capone that I've seen before include "Notorious criminal" , "Al - , notorious gangster" , "Hood" , "Chicago gangster, d. 1947" , "Italian-American gangster, d. 1947" .)