A large drink before sum (5)
I believe the answer is:
total
'sum' is the definition.
(I know that sum can be written as total)
'a large drink before' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'drink' becomes 'tot' (a small alcoholic drink).
'before' says to put letters next to each other.
'a'+'l'='al'
'al' after 'tot' is 'TOTAL'.
(Other definitions for total that I've seen before include "Not qualified" , "Come to" , "Aggregate, sum" , "Final tally" , "Entire sum" .)