“Sorry” will do, banal though it is (8)
I believe the answer is:
contrite
'sorry' is the definition.
('contrite' can be a synonym of 'sorry')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for contrite that I've seen before include "Regretful" , "Feeling remorse" , "Repentant and chastened" , "Repentant, remorseful" , "Feeling sorrow for sins" .)