Root beer, for one (4)
I believe the answer is:
soda
'root beer for one' is the definition.
(I know that root beer is a soda)
This is all the clue.
(Other definitions for soda that I've seen before include "Fizzy water, goes well with whiskey" , "Splash served with some spirits" , "for use with whisky?" , "Accompaniment to whisky" , "Sodium carbonate" .)