Feel regret (3)
I believe the answer is:
rue
'feel regret' 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 "Regret in a French street" , "Deplore" , "Regret; road (Fr.)" , "Regret or repent" , "Regret - wild shrub" .)