It's essential to reform cad and liar (7)
I believe the answer is:
radical
'it's essential' is the definition.
(thesaurus)
'reform cad and liar' is the wordplay.
'reform' indicates anagramming the letters (letters in a new form).
'and' means one lot of letters go next to another.
'cad' after 'liar' is 'liarcad'.
'liarcad' anagrammed gives 'RADICAL'.
'to' acts as a link.
(Other definitions for radical that I've seen before include "Dial car in order to be drastic" , "Political extremist" , "One holding strong reformist views" , "Primitive" , "One with rooted" .)