Reader's complaint flipped the old retinas out (9)
I believe the answer is:
eyestrain
'reader's complaint' is the definition.
'eyestrain' can be an answer for 'complaint' (I've seen this before). I am unsure of the 'reader's' bit.
'flipped the old retinas out' is the wordplay.
'flipped' says the letters should be written in reverse.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'ye' back-to-front is 'ey'.
'retinas' anagrammed gives 'estrain'.
'ey'+'estrain'='EYESTRAIN'
Can you help me to learn more?
(Other definitions for eyestrain that I've seen before include "What may give blurred sight" , "Vision tiredness" , "Discomfort of sightseers" , "Stress on mince pies (in the East End)?" , "Difficulty for reader" .)