Reserve opinion for matter (9)
I believe the answer is:
substance
'matter' is the definition.
('substance' can be a synonym of 'matter')
'reserve opinion' is the wordplay.
'reserve' becomes 'sub' (I've seen this in other clues).
'opinion' becomes 'stance' (I've seen this in other clues).
'sub'+'stance'='SUBSTANCE'
'for' acts as a link.
(Other definitions for substance that I've seen before include "wealth" , "power" , "Subject matter" , "Physical material" , "Basic matter" .)