Common mixer (5)
I believe the answer is:
tonic
'common mixer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tonic that I've seen before include "something nice and fresh" , "Keynote of a scale" , "Small drink of spirits" , "Carbonated mixer" , "Gin mixer - pick-me-up" .)