Extensive cover needed with account and IOUs (9)
I believe the answer is:
capacious
'extensive' is the definition.
(having a lot of capacity)
'cover needed with account and ious' is the wordplay.
'cover' becomes 'cap' (cap is a kind of cover).
'needed with' is a charade indicator (letters next to each other).
'account' becomes 'ac' (abbreviation for account).
'and' says to put letters next to each other.
'cap'+'ac'+'ious'='CAPACIOUS'
(Other definitions for capacious that I've seen before include "Roomy" , "Big" , "Commodious" , "Extensive" , "Voluminous" .)