Expressing regret (6)
I believe the answer is:
rueful
'expressing regret' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rueful that I've seen before include "Filled with regret" , "Feeling good-humoured regret" , "Penitent" , "'Contrite, remorseful (6)'" , "In a sorry state!" .)