Damaged, but OK in a gun (6)
I believe the answer is:
broken
'damaged' is the definition.
('broken' can be a synonym of 'damaged')
'ok in a gun' is the wordplay.
'in' indicates putting letters inside.
'a gun' becomes 'bren' (Bren is an example).
'ok' going within 'bren' is 'BROKEN'.
'but' is the link.
(Other definitions for broken that I've seen before include "Defective" , "If it's not ..., don't fix it" , "Busted, damaged" , "no longer working?" , "faulty" .)