Message Boris's school sent over (4)
I believe the answer is:
note
'message' is the definition.
(I know that message is a type of message)
'school sent over' is the wordplay.
'school' becomes 'eton' (famous UK public school).
'sent over' shows that the letters should be reversed in order.
'eton' backwards is 'NOTE'.
'boris's' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for note that I've seen before include "Pay close attention to" , "Observation" , "Tone of a short record" , "Fiver, say" , "Detect and remember" .)