Posh pupil in at one after correction (7)
I believe the answer is:
etonian
'posh pupil' is the definition.
'etonian' can be an answer for 'pupil' (I've seen this before). I am not sure about the 'posh' bit.
'in at one after correction' is the wordplay.
'after' is a charade indicator (letters next to each other).
'correction' is an anagram indicator (I've seen this in other clues).
'in'+'at'+'one'='inatone'
'inatone' with letters rearranged gives 'ETONIAN'.
Can you help me to learn more?
(Other definitions for etonian that I've seen before include "Public school kid" , "Relating to one particular school" , "student in establishment?" , "Pupil at posh English school" , "Alumnus of famous English school" .)