Medicine that may be mixed with gin (5)
I believe the answer is:
tonic
This clue is a double definition.
'medicine' is the first definition.
(tonic is a kind of medicine)
'that may be mixed with gin' is the second definition.
(I've seen this before)
(Other definitions for tonic that I've seen before include "Gin mixer - pick-me-up" , "Quinine water" , "something providing a boost" , "makes you feel good?" , "Stimulant" .)