A comparatively incisive crook (7)
I believe the answer is:
sharper
'a comparatively incisive crook' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'incisive crook' is the wordplay.
'incisive' becomes 'sharp' ('sharp' can be a synonym of 'incisive').
'crook' becomes 'er' (I can't justify this - if you can you should give a lot more credence to this answer).
'sharp'+'er'='SHARPER'
Can you help me to learn more?
(Other definitions for sharper that I've seen before include "Cheat" , "More acute" , "Card player" , "Swindler (at cards)" , "More acerbic" .)