Be repentant (3)
I believe the answer is:
rue
'be repentant' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rue that I've seen before include "way Parisians go about" , "Regret - wild shrub" , "Feel sorry" , "Repentant sort of shrub?" , "Feel bad" .)