Cheerful after small drink (3)
I believe the answer is:
sup
'drink' is the definition.
(to sup is to drink)
'cheerful after small' is the wordplay.
'cheerful' becomes 'up' (I've seen this in other clues).
'after' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'up' after 's' is 'SUP'.
(Other definitions for sup that I've seen before include "Drink; take a late meal" , "Drink; eat evening meal" , "Take food" , "Drink -- eat (with the devil?)" , "Imbibe" .)