Gin accompaniment (5)
I believe the answer is:
tonic
'gin accompaniment' 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 "fizz" , "that may be mixed with gin" , "Restorative that invigorates" , "shot in the arm" , "Quinine water" .)