Refuse stuff tossed back (4)
I believe the answer is:
marc
'refuse' is the definition.
(discarded material in winemaking)
'stuff tossed back' is the wordplay.
'stuff' becomes 'cram' (cramming is a kind of stuffing).
'tossed back' shows that the letters should be reversed in order.
'cram' reversed gives 'MARC'.
(Other definitions for marc that I've seen before include "Cram (anag.)" , "Drink" , "Brandy distilled from grape residue" , "Type of eau-de-vie made from residue of grapes" , "Spirit" .)