Revolutionary went off round city, one finds (8)
I believe the answer is:
detector
'one finds' is the definition.
'revolutionary went off round city' is the wordplay.
'revolutionary' says the letters should be written backwards.
'went off' becomes 'rotted'.
'round' means one lot of letters goes inside another.
'city' becomes 'ec' (the City of London postcode).
'rotted' backwards is 'dettor'.
'dettor' going around 'ec' is 'DETECTOR'.
(Other definitions for detector that I've seen before include "Sensor" , "Electronic device" , "Device for monitoring hazards" , "Device warning of, eg, smoke" .)