A gun each (6)
I believe the answer is:
apiece
'each' is the definition.
('apiece' can be a synonym of 'each')
'a gun' is the wordplay.
'gun' becomes 'piece' (piece is a kind of gun).
'a'+'piece'='APIECE'
(Other definitions for apiece that I've seen before include "For or by each one of group" , "Per person" , "A head" , "for everyone" .)