Incisiveness, let's say, of French revolutionary (4)
I believe the answer is:
edge
'incisiveness' is the definition.
(I've seen this before)
'let's say of french revolutionary' is the wordplay.
'let's say' becomes 'eg' (both can mean 'for example').
'of french' becomes 'de' ('of' in French).
'revolutionary' says the letters should be written backwards.
'eg'+'de'='egde'
'egde' reversed gives 'EDGE'.
(Other definitions for edge that I've seen before include "Upper hand" , "Rim - move gradually" , "Border or cutting side of knife" , "small advantage" , "Line where soething begins or ends" .)