Feel remorse about (3)
I believe the answer is:
rue
'feel remorse about' 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 "Feel regret for the herb" , "Bitter-tasting herb" , "Regret, be sorry for" , "French way" , "At one time, pity" .)