Message school sent back (4)
I believe the answer is:
note
'message' is the definition.
(note is a kind of message)
'school sent back' is the wordplay.
'school' becomes 'eton' (famous UK public school).
'sent back' says the letters should be written in reverse.
'eton' backwards is 'NOTE'.
(Other definitions for note that I've seen before include "Memo; pay attention to" , "Brief letter - distinction" , "Musical tone" , "Fiver, say" , "Some money - some music" .)