Fish supplement included in wine (7)
I believe the answer is:
haddock
'fish' is the definition.
(I know that haddock is a type of fish)
'supplement included in wine' is the wordplay.
'supplement' becomes 'add' (I've seen this before).
'included in' indicates putting letters inside.
'wine' becomes 'hock' (type of wine).
'add' going into 'hock' is 'HADDOCK'.
(Other definitions for haddock that I've seen before include "Fish, often smoked" , "Swimmer" , "Edible fish like small cod" , "Cod-like fish" , "Fish similar to but smaller than cod" .)