First note something reinvigorating (5)
I believe the answer is:
tonic
This clue is a double definition.
'first note' is the first definition.
(first note in a musical scale)
'something reinvigorating' is the second definition.
(Other definitions for tonic that I've seen before include "Accompaniment to gin?" , "something invigorating" , "Invigorating liquid" , "Small drink of spirits" , "makes you feel good?" .)