Boost unit in charge (5)
I believe the answer is:
tonic
'boost' is the definition.
(tonic can mean a stimulant or booster)
'unit in charge' is the wordplay.
'unit' becomes 'ton' (ton is a kind of unit).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'ton'+'ic'='TONIC'
(Other definitions for tonic that I've seen before include "__water; gin mixer" , "It goes with gin" , "Quinine drink; keynote" , "Key" , "do for musicians" .)