Observe revolutionary public school (4)
I believe the answer is:
eton
'public school' is the definition.
(Eton College)
'observe revolutionary' is the wordplay.
'observe' becomes 'note' (synonyms).
'revolutionary' shows that the letters should be reversed in order (letters revolve around).
'note' back-to-front is 'ETON'.
(Other definitions for eton that I've seen before include "Town opposite Windsor on the Thames" , "Collar - crop - jacket - school" , "British public school of note" , "Note sent up to English public school" , "Public school, established in 1440" .)