Drink to a large sum (5)
I believe the answer is:
total
'sum' is the definition.
(I know that sum can be written as total)
'drink to a large' is the wordplay.
'drink' becomes 'tot' (a small alcoholic drink).
'to' means one lot of letters go next to another.
'large' becomes 'L' (eg in clothes sizes).
'tot'+'a'+'l'='TOTAL'
(Other definitions for total that I've seen before include "Aggregate, sum" , "Entirety" , "Everything added together" , "final answer" , "Including all" .)