Weapon hidden in fellow's clothes (8)
I believe the answer is:
garments
'clothes' is the definition.
(I know this)
'weapon hidden in fellow's' is the wordplay.
'weapon' becomes 'arm' (synonyms).
'hidden in' is an insertion indicator.
'fellow's' becomes 'gent's' ('gent' can be a synonym of 'fellow').
'arm' going within 'gents' is 'GARMENTS'.
(Other definitions for garments that I've seen before include "clothes" , "Articles of clothing" .)