Narrator’s understood the reader’s older (6)
I believe the answer is:
senior
'older' is the definition.
('senior' can be a synonym of 'older')
I don't understand how the remainder of the clue works.
(Other definitions for senior that I've seen before include "Pupil in a higher form" , "High in rank" , "Elder (of two)" , "Older in years or higher in rank" , "Higher in rank or age" .)