Generous promises to pay after limit placed on account (9)
I believe the answer is:
capacious
'generous' is the definition.
(thesaurus)
'promises to pay after limit placed on account' is the wordplay.
'promises to pay' becomes 'IOU's' (promises to pay a debt).
'after' says to put letters next to each other.
'limit' becomes 'cap' (capping is a kind of limiting).
'placed on' says to put letters next to each other (in a down clue, some letters go on top of others).
'account' becomes 'ac' (abbreviation for account).
'cap'+'ac'='capac'
'ious' put after 'capac' is 'CAPACIOUS'.
(Other definitions for capacious that I've seen before include "Roomy" , "Big" , "Voluminous" , "Extensive" , "Commodious" .)