European certainly next to file complaint (9)
I believe the answer is:
eyestrain
'complaint' is the definition.
(I've seen this before)
'european certainly next to file' is the wordplay.
'european' becomes 'e' (abbreviation e.g. EU).
'certainly' becomes 'yes' (I've seen this before).
'next to' says to put letters next to each other.
'file' becomes 'train' (I am not sure about this - if you are sure you should believe this answer much more).
'e'+'yes'+'train'='EYESTRAIN'
Can you help me to learn more?
(Other definitions for eyestrain that I've seen before include "which makes it hard to see" , "this could require attention from ophthalmologist" , "Tension on the visual organ" , "tiredness of pupils" , "What may give blurred sight" .)