Basket supplying unboxed fizz! (6)
I believe the answer is:
hamper
'basket' is the definition.
(eg food hamper)
'unboxed fizz' is the wordplay.
'unboxed' means to look at the middle letters (middle letters are unwrapped or unboxed).
'fizz' becomes 'champers' (both are informal terms for champagne).
The middle of 'champers' is 'HAMPER'.
'supplying' is the link.
(Other definitions for hamper that I've seen before include "Impede the movement" , "basket case?" , "Slow up - basket" , "Food-basket" , "basket with a cover" .)