Regret, be sorry for (6)
I believe the answer is:
repent
I believe this clue is a double definition.
'regret' is the first definition.
(I know that regret can be written as repent)
'be sorry for' is the second definition.
(I've seen this before)
(Other definitions for repent that I've seen before include "Express remorse for sin" , "Feel sorry" , "Be ashamed" , "Feel sincere regret" , "Turn away from sin" .)