Agent with fine material for substitute (7)
I believe the answer is:
replace
'substitute' is the definition.
(synonyms)
'agent with fine material' is the wordplay.
'agent' becomes 'rep' (short for representative).
'with' is a charade indicator (letters next to each other).
'fine material' becomes 'lace'.
'rep'+'lace'='REPLACE'
'for' acts as a link.
(Other definitions for replace that I've seen before include "Substitute, supplant" , "Put back in original position" , "Put back or substitute" , "Put back where you found it" , "Put something back where it belongs" .)