Wine or tot knocked back by trainee (8)
I believe the answer is:
muscadet
'wine' is the definition.
(Muscadet is an example)
'tot knocked back by trainee' is the wordplay.
'tot' becomes 'sum' ('sum' can be a synonym of 'tot').
'knocked back' shows that the letters should be reversed in order.
'by' says to put letters next to each other.
'trainee' becomes 'cadet' (cadet is a kind of trainee).
'sum' backwards is 'mus'.
'mus'+'cadet'='MUSCADET'
'or' acts as a link.
(Other definitions for muscadet that I've seen before include "drink" , "Dry white wine from the Loire valley" , "one of the Whites" , "Dry French white wine" , "White wine variety" .)