Material witness right on the mark (10)
I believe the answer is:
seersucker
'material' is the definition.
(seersucker is a kind of material)
'witness right on the mark' is the wordplay.
'witness' becomes 'see' (I've seen this before).
'right' becomes 'r' (common abbreviation).
'on' is a charade indicator (letters next to each other).
'the mark' becomes 'sucker' ('sucker' can be a synonym of 'mark'**).
'see'+'r'+'sucker'='SEERSUCKER'
(Other definitions for seersucker that I've seen before include "Material" , "Cotton" , "Puckered fabric" .)