Surgical treatment on twitch in the eye (5)
I believe the answer is:
optic
'the eye' is the definition.
('optic' can be a synonym of 'eye')
'surgical treatment on twitch' is the wordplay.
'surgical treatment' becomes 'op' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'twitch' becomes 'tic' (tic is a kind of twitch).
'op'+'tic'='OPTIC'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for optic that I've seen before include "Of the eye - drinks dispenser" , "Pub measure" , "Device for measuring gin etc" , "Teacher's maybe dispensed with this" , "Relating to the eye" .)