Chap packing weapon in trousers, say (7)
I believe the answer is:
garment
'trousers say' is the definition.
(trousers is a kind of garment)
'chap packing weapon' is the wordplay.
'chap' becomes 'gent' (both can mean a man).
'packing' is an insertion indicator (some letters are packed inside).
'weapon' becomes 'arm' (synonyms).
'gent' going around 'arm' is 'GARMENT'.
'in' acts as a link.
(Other definitions for garment that I've seen before include "Grant me (anag.)" , "Something put on" , "Article of dress" , "coat maybe" , "Raiment" .)