Carbonated water (4)
I believe the answer is:
soda
'carbonated water' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for soda that I've seen before include "Drink containing carbonated water and sometimes flavouring" , "Accompaniment to whisky" , "Sodium carbonate" , "for use with whisky?" , "Fizzy water, goes well with whiskey" .)