Work on involuntary movement relating to the eye (5)
I believe the answer is:
optic
'relating to the eye' is the definition.
(I've seen this before)
'work on involuntary movement' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'on' means one lot of letters go next to another.
'involuntary movement' becomes 'tic' (I've seen this before).
'op'+'tic'='OPTIC'
(Other definitions for optic that I've seen before include "connected to the eye" , "Teacher's maybe coming through this" , "Pub measure" , "Spirit dispenser - of the eye" , "What could measure the spirits" .)