Gin mixer (5)
I believe the answer is:
tonic
'gin 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 "Restorative that invigorates" , "Quinine water" , "something nice and fresh" , "Stimulant" , "Keynote of a scale" .)