Operation on convulsive movement of eye (5)
I believe the answer is:
optic
'of eye' is the definition.
(I've seen this before)
'operation on convulsive movement' is the wordplay.
'operation' becomes 'op'.
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'convulsive movement' becomes 'tic' (a tic is a convulsive bodily movement).
'op'+'tic'='OPTIC'
(Other definitions for optic that I've seen before include "Body part" , "Visual" , "Of the eye - drinks dispenser" , "Like a sort of nerve" , "Spirits measurer" .)