Revolutionary secretly listens to argument (4)
I believe the answer is:
spat
'argument' is the definition.
(a spat is a disagreement or argument)
'revolutionary secretly listens to' is the wordplay.
'revolutionary' is a reversal indicator (letters revolve around).
'secretly listens to' becomes 'taps' (as in tapping a phone line).
'taps' written backwards gives 'SPAT'.
(Other definitions for spat that I've seen before include "Slight argument" , "Petty quarrel - gaiter" , "Gestured with contempt" , "Taps in a small quarrel" , "Spoke violently" .)