Refuse from crowd building up (4)
I believe the answer is:
marc
'refuse' is the definition.
(discarded material in winemaking)
'crowd building up' is the wordplay.
'crowd' becomes 'cram'.
'building up' is a reversal indicator.
'cram' in reverse letter order is 'MARC'.
'from' acts as a link.
(Other definitions for marc that I've seen before include "Chagall for one" , "Spirit" , "Brandy distilled from grape residue" , "Strong liquor" , "Sort of brandy made from the remains of pressed grapes" .)