Irritation right inside retina needing treatment (9)
I believe the answer is:
eyestrain
'irritation' is the definition.
(irritation in the eyes)
'right inside retina needing treatment' is the wordplay.
'right' becomes 'yes' (right can mean yes or okay).
'inside' means one lot of letters goes inside another.
'needing treatment' indicates an anagram (I've seen 'needs treatment' mean this (letters must be processed or treated)).
'retina' is an anagram of 'etrain'.
'yes' inserted inside 'etrain' is 'EYESTRAIN'.
(Other definitions for eyestrain that I've seen before include "Ocular tiredness" , "Reader's issue" , "Tension on the visual organ" , "pupils are involved in such trouble" , "A serenity leading to ocular stress" .)