Observes determined revolutionary (5)
I believe the answer is:
notes
'observes' is the definition.
('note' can be a synonym of 'observe')
'determined revolutionary' is the wordplay.
'determined' becomes 'set on' (I've seen this in another clue).
'revolutionary' says the letters should be written in reverse (letters revolve around).
'seton' reversed gives 'NOTES'.
(Other definitions for notes that I've seen before include "Short written messages" , "registers?" , "Memoranda" , "Observes; short comments" , "Lines" .)