It's essential to reform a cad and a liar (7)
I believe the answer is:
radical
'it's essential' is the definition.
(thesaurus)
'reform a cad and a liar' is the wordplay.
'reform' indicates anagramming the letters (letters in a new form).
'and' is a charade indicator (letters next to each other).
'cad' after 'liar' is 'liarcad'.
'liarcad' anagrammed gives 'RADICAL'.
'to' is the link.
(Other definitions for radical that I've seen before include "Drastic or thorough as changes may be" , "Fundamental - non-conformist" , "(One) favouring fundamental changes" , "One who favours thoroughgoing reform" , "Progressive - car dial (anag)" .)