Ditch books about American leader? That's incisive (9)
I believe the answer is:
trenchant
'incisive' is the definition.
(thesaurus)
'ditch books about american leader?' is the wordplay.
'ditch' becomes 'trench' (trench is a kind of ditch**).
'books' becomes 'nt' (New Testament).
'about' is a charade indicator (letters next to each other).
'leader?' suggests taking the first letters (I've seen 'leader' mean this).
The first letter of 'american' is 'a'.
'nt' after 'a' is 'ant'.
'trench'+'ant'='TRENCHANT'
'that's' is the link.
(Other definitions for trenchant that I've seen before include "blunt" , "Forthright, vigorous" , "Sharp" , "Clear-cut and hard-hitting" , "Cutting, forthright" .)