Regret being cruel at heart (3)
I believe the answer is:
rue
'regret' is the definition.
(to rue something is to regret it)
'cruel at heart' is the wordplay.
'at heart' says the answer is hidden in the clue (letters hidden in the heart of the clue).
'RUE' can be found hidden inside 'cruel'.
'being' is the link.
(Other definitions for rue that I've seen before include "Cry" , "The 'herb of grace'" , "show you're sorry!" , "Deplore" , "Bitter-tasting herb" .)