Great success concerning voucher (6)
I believe the answer is:
coupon
'voucher' is the definition.
(synonyms)
'great success concerning' is the wordplay.
'great success' becomes 'coup' (coup can mean an impressive feat or achievement).
'concerning' becomes 'on'.
'coup'+'on'='COUPON'
(Other definitions for coupon that I've seen before include "betting slip" , "Voucher or ticket" , "Entry form" , "means to buy goods" , "Token" .)