Spread complete by start of banquet (6)
I believe the answer is:
butter
'spread' is the definition.
(butter is a spread)
'complete by start of banquet' is the wordplay.
'complete' becomes 'utter' (both can mean total).
'by' is a charade indicator (letters next to each other).
'start of' says to take the initial letters.
The first letter of 'banquet' is 'b'.
'utter' put after 'b' is 'BUTTER'.
(Other definitions for butter that I've seen before include "Milk product" , "Spread derived from cream" , "Spread for bread" , "Dairy spread" , "Dairy product" .)