Cold warmth? That's a fraud (5)
I believe the answer is:
cheat
'that's a fraud' is the definition.
(cheat is a kind of fraud)
'cold warmth?' is the wordplay.
'cold' becomes 'C' (abbreviation, eg on taps).
'warmth?' becomes 'heat' (synonyms).
'c'+'heat'='CHEAT'
(Other definitions for cheat that I've seen before include "Skin" , "Play false" , "Fraud" , "Rogue" , "Hoodwink" .)