Beat a gangster (6)
I believe the answer is:
capone
'gangster' is the definition.
(Al Capone)
'beat a' is the wordplay.
'beat' becomes 'cap'.
'a' becomes 'one'.
'cap'+'one'='CAPONE'
(Other definitions for capone that I've seen before include "Al ..., US gangster" , "Hood" , "Al - , notorious gangster" , "Chicago gangster, d. 1947" , "US gangster (St Valentine's Day massacre)" .)