Master stroke on finding the voucher (6)
I believe the answer is:
coupon
'the voucher' is the definition.
('coupon' can be a synonym of 'voucher')
'master stroke on' is the wordplay.
'master stroke' becomes 'coup' (I've seen this in another clue).
'coup'+'on'='COUPON'
'finding' acts as a link.
(Other definitions for coupon that I've seen before include "means to buy goods" , "Token" , "betting slip" , "Entry form" , "Voucher or ticket" .)