Seeing difficulty certainly, retinue on message in the end (9)
I believe the answer is:
eyestrain
'seeing difficulty' is the definition.
(difficulty when seeing)
'certainly retinue on message in the end' is the wordplay.
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'retinue' becomes 'train' (both can mean people following an important person).
'on' says to put letters next to each other.
'in the end' indicates one should take the final letters.
The last letter of 'message' is 'e'.
'yes'+'train'='yestrain'
'yestrain' put after 'e' is 'EYESTRAIN'.
(Other definitions for eyestrain that I've seen before include "puts pressure on pupils" , "Difficulty for reader" , "Tiredness when reading" , "Reader's issue" , "Stress on mince pies (in the East End)?" .)