A public school of unusual note (4)
I believe the answer is:
eton
'a public school' is the definition.
(Eton College)
'unusual note' is the wordplay.
'unusual' is an anagram indicator.
'note' anagrammed gives 'ETON'.
'of' acts as a link.
(Other definitions for eton that I've seen before include "old school" , "Posh English school of note" , "College, collar or crop" , "School near Slough" , "Place for learning" .)