Mixer at a bar (5)
I believe the answer is:
tonic
'mixer at a bar' 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 "that may be mixed with gin" , "Invigorating liquid" , "Accompaniment to gin?" , "Invigorating medicine - keynote" , "something providing a boost" .)