Have remorse about a piece of cruelty (3)
I believe the answer is:
rue
'have remorse' is the definition.
(I've seen this in another clue)
'a piece of cruelty' is the wordplay.
'a piece of' indicates the answer is hidden within the clue.
'RUE' is hidden within 'cruelty'.
'about' is the link.
(Other definitions for rue that I've seen before include "Regret the bitter herb" , "Feel remorse" , "Swann's way?" , "Regret, be sorry for" , "Bitterly regret - plant" .)