Note shot in the arm (5)
I believe the answer is:
tonic
This clue is a double definition.
'note' is the first definition.
(type of note in a musical scale)
'shot in the arm' is the second definition.
(both can mean something which stimulates)
(Other definitions for tonic that I've seen before include "Stimulant" , "shot in the arm" , "something invigorating" , "fizz" , "something medicinal" .)