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' says the letters should be written in reverse (letters revolve around).
'note' in reverse letter order 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" .)