Regret cruelty to an extent (3)
I believe the answer is:
rue
'regret' is the definition.
(to rue something is to regret it)
'cruelty to an extent' is the wordplay.
'to an extent' indicates a hidden word.
'RUE' is hidden in the letters of 'cruelty'.
(Other definitions for rue that I've seen before include "show regret" , "Regret the bitter herb" , "Lament; a herb" , "Wish not to have been" , "Regret, be sorry for" .)