Letter school returned (4)
I believe the answer is:
note
'letter' is the definition.
(note is a kind of letter)
'school returned' is the wordplay.
'school' becomes 'eton' (famous UK public school).
'returned' says the letters should be written in reverse.
'eton' backwards is 'NOTE'.
(Other definitions for note that I've seen before include "Observe; short letter" , "A, B, C, D, F or G" , "Annotation; tone" , "Put down" , "mention" .)