Message posh school sent over (4)
I believe the answer is:
note
'message' is the definition.
(I know that message can be written as note)
'posh school sent over' is the wordplay.
'posh school' becomes 'Eton'.
'sent over' shows that the letters should be reversed in order.
'eton' written backwards gives 'NOTE'.
(Other definitions for note that I've seen before include "Observation" , "register" , "mention" , "Write down" , "Tone of distinction" .)