Snooping on revolutionary rat (6)
I believe the answer is:
nosing
'snooping' is the definition.
(I've seen this before)
'on revolutionary rat' is the wordplay.
'revolutionary' is a reversal indicator (letters revolve around).
'rat' becomes 'sing' (both can mean to inform on someone).
'on' written backwards gives 'no'.
'no'+'sing'='NOSING'
(Other definitions for nosing that I've seen before include "Sign on (anag)" , "Moving slowly" , "Being inquisitive; advancing slowly" , "snooping" , "Defeating by a narrow margin" .)