Brief letter school sent back (4)
I believe the answer is:
note
'brief letter' is the definition.
(I've seen this before)
'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 "Fiver, say" , "Memo - sound - fame" , "Take this to heed" , "Money-carrier > hit repeatedly by 'charmingly sweet' singer" , "Short message; piano key" .)