Feel regret about (3)
I believe the answer is:
rue
'feel regret about' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rue that I've seen before include "Bitterly regret - plant" , "Regret in a French street" , "repentance" , "Regret the bitter herb" , "The 'herb of grace'" .)