Letter from school turning up (4)
I believe the answer is:
note
'letter' is the definition.
(note is a kind of letter)
'school turning up' is the wordplay.
'school' becomes 'eton' (famous UK public school).
'turning up' says the letters should be written backwards.
'eton' backwards is 'NOTE'.
'from' is the link.
(Other definitions for note that I've seen before include "Fiver, say" , "Observe the musical pitch." , "Jotting" , "Symbol in a score" , "Eg, minim" .)