Letter to sorcerer causes involuntary movement (6)
I believe the answer is:
twitch
'involuntary movement' is the definition.
(an involuntary jerking movement)
'letter to sorcerer' is the wordplay.
'letter' becomes 't' ().
'to sorcerer' becomes 'witch' (I've seen this before).
't'+'witch'='TWITCH'
'causes' is the link.
(Other definitions for twitch that I've seen before include "Jerky reaction" , "Sudden jerky movement; pang" , "Move spasmodically" , "jump" , "by implication, is to look for birds" .)