Work with convulsive twitch of the eye (5)
I believe the answer is:
optic
'of the eye' is the definition.
(I've seen this before)
'work with convulsive twitch' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'with' means one lot of letters go next to another.
'convulsive twitch' becomes 'tic' (nervous tic).
'op'+'tic'='OPTIC'
(Other definitions for optic that I've seen before include "Picot (anag.)" , "Eye - spirit measure" , "Device for measuring out spirits" , "Relating to vision" , "Spirits measurer" .)