Drink boost (5)
I believe the answer is:
tonic
This clue is a double definition.
'drink' is the first definition.
(tonic is a kind of drink)
'boost' is the second definition.
(tonic can mean a stimulant or booster)
(Other definitions for tonic that I've seen before include "Gin mixer - pick-me-up" , "Key" , "Keynote of a scale" , "fizz" , "Invigorating liquid" .)