Criminal male beheaded crustacean (7)
I believe the answer is:
mobster
'criminal' is the definition.
(mobster is a kind of criminal)
'male beheaded crustacean' is the wordplay.
'male' becomes 'm' (common abbreviation).
'beheaded' means to remove the first letter (the head of the word is removed).
'crustacean' becomes 'lobster' (lobster is a kind of crustacean).
'lobster' with its initial letter taken off is 'obster'.
'm'+'obster'='MOBSTER'
(Other definitions for mobster that I've seen before include "Mafioso" , "Hood" , "Gangster" , "family man" , "Hoodlum" .)