Work on nervous reaction of the eye (5)
I believe the answer is:
optic
'of the eye' is the definition.
(I've seen this before)
'work on nervous reaction' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'on' means one lot of letters go next to another.
'nervous reaction' becomes 'tic' (I've seen this in other clues).
'op'+'tic'='OPTIC'
(Other definitions for optic that I've seen before include "Of the eye - drinks dispenser" , "What could measure the spirits" , "bar assistant" , "affecting vision?" , "Of sight or the eye" .)