Deep regret (7)
I believe the answer is:
remorse
'deep regret' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for remorse that I've seen before include "it may bite" , "Sorrow for wrongs committed" , "Feeling of regret for wrongdoing" , "Sorrow for wrong committed" , "Regret or sorrow for wrong-doing" .)