Wine: quantity upset over trainee (8)
I believe the answer is:
Here is my best explanation:
'wine' is the definition.
(Muscadet is an example)
'quantity upset over trainee' is the subsidiary indication.
'quantity' becomes 'sum' (sum is a kind of quantity**).
'upset' shows that the letters should be reversed in order.
'over' means one lot of letters go next to another.
'trainee' becomes 'cadet' (cadet is a kind of trainee).
'sum' reversed gives 'mus'.
(Other definitions for muscadet that I've seen before include "Dry white wine from the Loire valley", "Dry French white wine", "White wine variety", "drink", "one of the Whites", "French dry white wine" and "A dry white wine".)