Gangster’s headgear (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)
'headgear' is the second definition.
(hood is a kind of headgear)
(Other definitions for hood that I've seen before include "Part of subfusc" , "Thomas -, Eng. poet" , "Cover < the upper part < of tough" , "Outlaw and gangster" , "A cover; Bismarck sank it, 1941" .)