Son finished drink (3)
I believe the answer is:
sup
'drink' is the definition.
(to sup is to drink)
'son finished' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'finished' becomes 'up' (eg 'time's up' means the time is finished).
's'+'up'='SUP'
(Other definitions for sup that I've seen before include "Drink; take a late meal" , "Drink -- eat (with the devil?)" , "Drink; eat evening meal" , "Drink; take an evening meal" , "Imbibe" .)