Drink most of the ale, or all of it! (5)
I believe the answer is:
total
'all of it' is the definition.
(I've seen this in another clue)
'drink most of the ale' is the wordplay.
'drink' becomes 'tot' (a small alcoholic drink).
'most of' means to remove the last letter.
'ale' with its final letter taken off is 'al'.
'tot'+'al'='TOTAL'
'or' acts as a link.
(Other definitions for total that I've seen before include "wreck" , "Aggregate amount" , "Ruin" , "Come to" , "Damage beyond repair" .)