Tory retains too much material (6)
I believe the answer is:
cotton
'material' is the definition.
(cotton is a kind of material)
'tory retains too much' is the wordplay.
'tory' becomes 'con' (abbreviation for Conservative).
'retains' is an insertion indicator.
'too much' becomes 'ott' (short for 'over the top' - informal British).
'con' enclosing 'ott' is 'COTTON'.
(Other definitions for cotton that I've seen before include "Kind of thread or fabric" , "Soft white substance used to make fabric - in song, Old Man River don't plant it" , "Substance like fine wool" , "Natural textile fibre" , "Fibre including kapok" .)