A drink then, Father (4)
I believe the answer is:
soda
'a drink' is the definition.
(I know that soda is a type of soft drink)
'then father' is the wordplay.
'then' becomes 'so' (both can mean 'therefore' or 'in that case').
'father' becomes 'da' (I've seen this in other clues).
'so'+'da'='SODA'
(Other definitions for soda that I've seen before include "for use with whisky?" , "Variety of pop" , "Accompaniment to whisky" , "Type of bread, water or fountain" , "Fizzy drink - chemical compound" .)