Old Testament character with hardened heart (4)
I believe the answer is:
seth
'old testament character' is the definition.
(I've seen this before)
'with hardened heart' is the wordplay.
'with' is a charade indicator (letters next to each other).
'hardened' becomes 'set' (synonyms).
'heart' becomes 'h' (card game abbreviation).
'set'+'h' is 'SETH'.
(Other definitions for seth that I've seen before include "Third son of Adam and Eve" , "A Starkadder" , "said to sound archaic" , "Boy's name" , "Adam and Eve's third son" .)