John the singer is left in school (5)
I believe the answer is:
elton
'john the singer' is the definition.
(I've seen this in another clue)
'left in school' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'school' becomes 'eton' (famous UK public school).
'l' put within 'eton' is 'ELTON'.
'is' is the link.
(Other definitions for elton that I've seen before include "..... Jonn (musician)" , "Sir ___ John, singer-songwriter" , "- John, British singer" , "One John" , "First name of singer/songwriter born Reginald Dwight in 1947" .)