Medicine working to stop twitch (5)
I believe the answer is:
tonic
'medicine' is the definition.
(tonic is a kind of medicine)
'working to stop twitch' is the wordplay.
'working' becomes 'on'.
'to stop' indicates putting letters inside.
'twitch' becomes 'tic' (tic is a kind of twitch).
'on' placed inside 'tic' is 'TONIC'.
(Other definitions for tonic that I've seen before include "It goes with gin" , "something nice and fresh" , "Restorative that invigorates" , "Invigorating drink" , "__water; gin mixer" .)