Part of a fast-food combo (4)
I believe the answer is:
soda
'part of a fast-food combo' 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 "kind of water" , "for use with whisky?" , "pressurised liquid" , "Whisky mixer" , "Type of bread, water or fountain" .)