Bad debts covered by firm; investor finally cheers up (9)
I believe the answer is:
atrocious
'bad' is the definition.
('atrocious' can be similar in meaning to 'bad')
'debts covered by firm investor finally cheers up' is the wordplay.
'debts' becomes 'ious'.
'covered by' means one lot of letters go next to another (on top of in a down clue).
'firm investor' becomes 'roc' (I am not sure about this - if you are sure you should believe this answer much more).
'finally' says to put letters next to each other.
'cheers up' becomes 'at' (I've seen this in other clues).
'ious' put after 'roc' is 'rocious'.
'rocious' put after 'at' is 'ATROCIOUS'.
(Other definitions for atrocious that I've seen before include "Horrifying" , "Extremely bad" , "Dreadful" , "Horrifyingly wicked" , "Shocking" .)