A decent chap, mostly chasing fine material (6)
I believe the answer is:
fabric
'material' is the definition.
('fabric' can be a synonym of 'material')
'a decent chap mostly chasing fine' is the wordplay.
'decent chap' becomes 'brick' (I've seen this in other clues).
'mostly' means to remove the last letter.
'chasing' says to put letters next to each other.
'fine' becomes 'f' (abbreviation).
'brick' with its final letter taken away is 'bric'.
'a'+'bric'='abric'
'abric' after 'f' is 'FABRIC'.
(Other definitions for fabric that I've seen before include "Material; structure" , "Manufactured cloth" , "Silk or satin, say" , "Woven, knitted or felted cloth" , "Material in general" .)