An old tale about the business can be asserted (10)
I believe the answer is:
affirmable
'an old tale about the business can be asserted' is the definition.
I can't tell whether this defines the answer.
'tale about the business can be asserted' is the wordplay.
I cannot really understand how this works, but
'about' could be 'a' (abbreviation in Chambers) and 'a' is located in the answer.
'business' could be 'firm' (firm is a kind of business) and 'firm' is found within the answer.
'tale' could be 'fable' (I've seen this in another clue) and 'fable' is present in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?