Gun man (5)
I believe the answer is:
piece
I believe this clue is a double definition.
'gun' is the first definition.
(piece is a kind of gun)
'man' is the second definition.
(synonyms)
(Other definitions for piece that I've seen before include "Castle, perhaps" , "Musical composition" , "See 3" , "Chessman - part - firearm" , "constituent" .)