Key boost (5)
I believe the answer is:
tonic
This clue is a double definition.
'key' is the first definition.
(I've seen this before)
'boost' is the second definition.
(tonic can mean a stimulant or booster)
(Other definitions for tonic that I've seen before include "Fillip" , "Key" , "It goes with gin" , "Keynote of a scale" , "shot in the arm" .)