Vehement cheat — PM, I fancy (8)
I believe the answer is:
emphatic
'vehement' is the definition.
(thesaurus)
'cheat pm i fancy' is the wordplay.
'fancy' is an anagram indicator.
'cheat'+'pm'+'i'='cheatpmi'
'cheatpmi' is an anagram of 'EMPHATIC'.
(Other definitions for emphatic that I've seen before include "Striking" , "being stressed" , "speaking firmly" , "Categorical" , "Forceful in utterance" .)