"Sorry" will do, banal though it is (8)
I believe the answer is:
contrite
'sorry' is the definition.
(synonyms)
'do banal though it is' is the wordplay.
I cannot really understand how this works, but
'do' could be 'con' (both can mean to defraud) and 'con' is located in the answer.
'banal' could be 'trite' (I've seen this before) and 'trite' is present in the answer.
No letters remain.
This explanation may well be incorrect...
'will' acts as a link.
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" .)