A day before a party, providing booze (11)
I believe the answer is:
amontillado
'booze' is the definition.
(I've seen this before)
'a day before a party' is the wordplay.
'day' becomes 'mon'.
'before' becomes 'till' (**).
'party' becomes 'do' (do is a kind of party).
'a'+'mon'+'till'+'a'+'do'='AMONTILLADO'
'providing' acts as a link.
(Other definitions for amontillado that I've seen before include "Kind of sherry" , "Drink" , "Type of sherry" , "Here's sherry" , "Pale sherry" .)