Gangster's cowl (4)
I believe the answer is:
hood
I believe this clue is a double definition.
'gangster's' is the first definition.
(I've seen this before)
'cowl' is the second definition.
(I know that cowl can be written as hood)
(Other definitions for hood that I've seen before include "Bonnet (US) - gangster (US)" , "Protective cover - violent criminal (slang)" , "academic garment" , "this could provide cover" , "Cowl -- violent criminal (slang)" .)