Mixer drink (5)
'mixer drink' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for soda that I've seen before include "Soft drink" , ""Splashed" drinks mixer" , "Whisky mixer" , "Accompaniment to whisky" , "Sodium carbonate" .)
(Other definitions for tonic that I've seen before include "Carbonated mixer" , "Key" , "fizz" , "Restorative that invigorates" , "Invigorating medicine - keynote" .)