Whisky mixer (4)
I believe the answer is:
soda
'whisky mixer' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for soda that I've seen before include "Splash served with some spirits" , "for use with whisky?" , "Variety of pop" , "Fizzy water, goes well with whiskey" , "Mixer drink" .)