Reserve player's attitude when facing reality (9)
I believe the answer is:
substance
'reality' is the definition.
(thesaurus)
'reserve player's attitude when facing' is the wordplay.
'reserve player' becomes 'sub' (substitute player in sports).
'attitude when facing' becomes 'stance' (stance assumed when facing a particular task).
'sub'+'stance'='SUBSTANCE'
(Other definitions for substance that I've seen before include "Subject matter" , "Physical matter" , "Physical material" , "Property" , "power" .)