Shots in the arm working to stop spasms (6)
I believe the answer is:
tonics
'shots in the arm' is the definition.
(a shot in the arm is a tonic or stimulant)
'working to stop spasms' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'to stop' is an insertion indicator.
'spasms' becomes 'tics' (tic is a kind of spasm).
'on' inserted within 'tics' is 'TONICS'.
(Other definitions for tonics that I've seen before include "Keys" , "Pick-me-ups" , "contents of medicine cupboards" , "Pick-me-up drinks" , "Medicines or drinks with invigorating effects" .)