Tonic's partner (3)
I believe the answer is:
gin
'tonic's partner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gin that I've seen before include "Juniper-flavoured drink" , "Spirit; trap" , "Spirit drink" , "It could be used for drinking" , "It often gets mixed up with this" .)