Drink when drinking party returns (4)
I believe the answer is:
soda
'drink' is the definition.
(soda is a kind of drink)
'when drinking party returns' is the wordplay.
'when' becomes 'as'.
'drinking' indicates putting letters inside.
'party' becomes 'do' (do is a kind of party).
'returns' shows that the letters should be reversed in order.
'as' enclosing 'do' is 'ados'.
'ados' in reverse letter order is 'SODA'.
(Other definitions for soda that I've seen before include "fizz" , "Accompaniment to whisky" , "In the US, a sweet fizzy drink" , "Alkaline substance used in cleaning, drinks etc" , "Type of bread, water or fountain" .)