Sort of school greeting being heard (4)
I believe the answer is:
high
'sort of school' is the definition.
(high school)
'greeting being heard' is the wordplay.
'greeting' becomes 'hi' (hi is a kind of greeting).
'being heard' shows a homophone (sound like).
'hi' is a homophone of 'HIGH'.
(Other definitions for high that I've seen before include "First-rate - drunk" , "State of euphoria" , "Large, complex (and shady?) money transactions" , "Elevated; euphoric" , "Important; drugged" .)