Spy knows Russian ambassador never starts revolution (4)
I believe the answer is:
nark
'spy' is the definition.
(I've seen this before)
'knows russian ambassador never starts revolution' is the wordplay.
'starts' suggests taking the first letters.
'revolution' shows that the letters should be reversed in order.
The initial letters of 'knows russian ambassador never' is 'kran'.
'kran' written backwards gives 'NARK'.
(Other definitions for nark that I've seen before include "Informer - irritate" , "Annoy; informer (sl.)" , "cause irritation" , "a nuisance down under" , "Annoy - informant" .)