Reserve position in matter (9)
I believe the answer is:
substance
'matter' is the definition.
(synonyms)
'reserve position' is the wordplay.
'reserve' becomes 'sub' (I've seen this in other clues).
'position' becomes 'stance' (stance is a kind of position).
'sub'+'stance'='SUBSTANCE'
'in' is the link.
(Other definitions for substance that I've seen before include "Physical material" , "Subject matter" , "Physical matter" , "power" , "Property" .)