Wine: quantity upset over trainee (8)
I believe the answer is:
muscadet
'wine' is the definition.
(Muscadet is an example)
'quantity upset over trainee' is the wordplay.
'quantity' becomes 'sum' (sum is a kind of quantity**).
'upset' is a reversal indicator.
'over' is a charade indicator (letters next to each other).
'trainee' becomes 'cadet' (cadet is a kind of trainee).
'sum' backwards is 'mus'.
'mus'+'cadet'='MUSCADET'
(Other definitions for muscadet that I've seen before include "White wine variety" , "Dry white wine from the Loire valley" , "drink" , "one of the Whites" , "Dry French white wine" .)