Advance policy for prosperity (9)
I believe the answer is:
substance
'prosperity' is the definition.
(thesaurus)
'advance policy' is the wordplay.
'advance' becomes 'sub' (I've seen this in other clues).
'policy' becomes 'stance' (I've seen this before).
'sub'+'stance'='SUBSTANCE'
'for' is the link.
(Other definitions for substance that I've seen before include "Physical material" , "Matter or material that a thing is made of" , "Property" , "Basic matter" , "wealth" .)