Material that's fine on a mostly decent chap (6)
I believe the answer is:
fabric
'material' is the definition.
('fabric' can be a synonym of 'material')
'fine on a mostly decent chap' is the wordplay.
'fine' becomes 'f' (abbreviation).
'on' is a charade indicator (letters next to each other).
'mostly' means to remove the last letter.
'decent chap' becomes 'brick' (I've seen this in other clues).
'brick' with its final letter taken off is 'bric'.
'f'+'a'+'bric'='FABRIC'
'that's' acts as a link.
(Other definitions for fabric that I've seen before include "Woven textile" , "handiwork?" , "(Texture of) cloth" , "Cotton or tweed, say" , "Underlying structure" .)