Informal letter public school circulated (4)
I believe the answer is:
note
'informal letter' is the definition.
(I've seen this before)
'public school circulated' is the wordplay.
'public school' becomes 'eton' (Eton College).
'circulated' shows that the letters should be reversed in order (I've seen 'circulating' mean this).
'eton' back-to-front is 'NOTE'.
(Other definitions for note that I've seen before include "Sound, record" , "Write down" , "Traitor" , "Record - piece of music" , "Fiver, say" .)