Ring starts involuntary movement of the ear (4)
I believe the answer is:
otic
'of the ear' is the definition.
(I've seen this in another clue)
'ring starts involuntary movement' is the wordplay.
I cannot quite see how this works, but
'ring' could be 'o' (letter resembling a ring) and 'o' is found in the answer.
'movement' could be 'tic' (an involuntary physical movement) and 'tic' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for otic that I've seen before include "about hearing" , "of concern to the listener" , "relating to area incorporating canal" , "of lugs" , "Relating to the ear" .)