Do penance (5)
I believe the answer is:
atone
'do penance' is the definition.
(to do penance is to atone for something)
This is the entire clue.
(Other definitions for atone that I've seen before include "Make reparation for a misdeed" , "Recompense" , "Expiate" , "Turn away from sin and do penance" , "remedy" .)