Revolutionary went off round city, one finds (8)
I believe the answer is:
Here is my best explanation:
'one finds' is the definition.
'revolutionary went off round city' is the wordplay.
'revolutionary' says the letters should be written in reverse.
'went off' becomes 'rotted'.
'round' indicates putting letters inside.
'city' becomes 'ec' (the City of London postcode).
'rotted' back-to-front is 'dettor'.
'dettor' placed around 'ec' is 'DETECTOR'.
(Other definitions for detector that I've seen before include "Sensor", "Electronic device", "Device for monitoring hazards" and "Device warning of, eg, smoke".)