Amount given back to younger son to produce wine (8)
I believe the answer is:
muscadet
'wine' is the definition.
(Muscadet is an example)
'amount given back to younger son' is the wordplay.
'amount' becomes 'sum' ('sum' can be a synonym of 'amount').
'given back' is a reversal indicator.
'to younger son' becomes 'cadet' (archaic).
'sum' back-to-front is 'mus'.
'mus'+'cadet'='MUSCADET'
'to produce' is the link.
(Other definitions for muscadet that I've seen before include "drink" , "one of the Whites" , "Dry French white wine" , "White wine variety" , "Dry white wine from the Loire valley" .)