Regret having cruel heart (3)
I believe the answer is:
rue
'regret' is the definition.
(to rue something is to regret it)
'cruel heart' is the wordplay.
'heart' indicates the central letters.
The middle letters of 'cruel' are 'RUE'.
'having' is the link.
(Other definitions for rue that I've seen before include "Regret or repent" , "Regret, be sorry for" , "Lament; a herb" , "The 'herb of grace'" , "Regret - wild shrub" .)