Work on twitch of the eye (5)
I believe the answer is:
optic
'of the eye' is the definition.
(I've seen this before)
'work on twitch' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'on' means one lot of letters go next to another.
'twitch' becomes 'tic' (tic is a kind of twitch).
'op'+'tic'='OPTIC'
(Other definitions for optic that I've seen before include "Body part" , "Pub drinks dispenser" , "Spirit dispenser - of the eye" , "of a viewer?" , "Relating to the eyes" .)