Regret being endlessly cruel (3)
I believe the answer is:
rue
'regret' is the definition.
(I've seen this before)
'endlessly cruel' is the wordplay.
'endlessly' indicates the central letters (the ends of the words are removed).
The centre of 'cruel' is 'RUE'.
'being' is the link.
(Other definitions for rue that I've seen before include "At one time, pity" , "Regret in a French street" , "Repentant sort of shrub?" , "Regret - wild shrub" , "repentance" .)