Scotch's partner (4)
I believe the answer is:
soda
'scotch's partner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for soda that I've seen before include "Alkaline substance used in cleaning, drinks etc" , "kind of water" , "Variety of pop" , "In the US, a sweet fizzy drink" , ""Splashed" drinks mixer" .)