Message posh school sent up (4)
I believe the answer is:
note
'message' is the definition.
(I know that message is a type of message)
'posh school sent up' is the wordplay.
'posh school' becomes 'Eton'.
'sent up' is a reversal indicator.
'eton' backwards is 'NOTE'.
(Other definitions for note that I've seen before include "Brief letter - distinction" , "A, B or C, for example - short message" , "Short informal letter" , "Observe; paper money" , "held by piano teacher" .)