Amount given back to younger son to produce wine (8)
I believe the answer is:
Here is my best explanation:
'wine' is the definition.
(Muscadet is an example)
'amount given back to younger son' is the subsidiary indication.
'amount' becomes 'sum' ('sum' can be a synonym of 'amount'**).
'given back' is a reversal indicator.
'to younger son' becomes 'cadet' (archaic).
'sum' written backwards gives 'mus'.
'to produce' is the link.
(Other definitions for muscadet that I've seen before include "one of the Whites", "French dry white wine", "A dry white wine", "Dry French white wine", "White wine variety", "Dry white wine from the Loire valley" and "drink".)