Trainee with problem knocked back wine (8)
I believe the answer is:
muscadet
'wine' is the definition.
(Muscadet is an example)
'trainee with problem knocked back' is the wordplay.
'trainee' becomes 'cadet' (cadet is a kind of trainee).
'with' is a charade indicator (letters next to each other).
'problem' becomes 'sum' (arithmetic problem).
'knocked back' shows that the letters should be reversed in order.
'sum' written backwards gives 'mus'.
'cadet' after 'mus' is 'MUSCADET'.
(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" .)