Singer John learner in posh school (5)
I believe the answer is:
elton
'singer john' is the definition.
(I've seen this in another clue)
'learner in posh school' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'in' is an insertion indicator.
'posh school' becomes 'Eton'.
'l' inserted within 'eton' is 'ELTON'.
(Other definitions for elton that I've seen before include "Ben --; -- John" , "Sir ___ John, singer-songwriter" , "- John, British singer" , "One John" , "Sir - - John, singer songwriter" .)