Regrets making heartless laws (4)
I believe the answer is:
rues
'regrets' is the definition.
(I've seen this before)
'heartless laws' is the wordplay.
'heartless' means to remove the middle letters.
'laws' becomes 'rules' (rule is a kind of law).
'rules' with its centre taken out is 'RUES'.
'making' is the link.
(Other definitions for rues that I've seen before include "Regrets, to be sure" , "Is sorry for - plants" , "Bemoans" .)