Be sorry for a piece of cruelty (3)
I believe the answer is:
rue
'be sorry for' is the definition.
(I've seen this in another clue)
'a piece of cruelty' is the wordplay.
'a piece of' says the answer is hidden in the clue.
'RUE' is hidden in the letters of 'cruelty'.
(Other definitions for rue that I've seen before include "Bemoan" , "Repentant sort of shrub?" , "Regret - wild shrub" , "Feel remorse" , "Feel regret for the herb" .)