Arrangement with bank manager finally accepted by maiden? Silly! (9)
I believe the answer is:
overdraft
'arrangement with bank' is the definition.
(I've seen this before)
'manager finally accepted by maiden? silly' is the wordplay.
'manager finally' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'accepted by' is an insertion indicator.
'maiden?' becomes 'over' (maiden is a kind of over).
'silly' becomes 'daft' (I've seen this before).
'over'+'daft'='overdaft'
'r' placed into 'overdaft' is 'OVERDRAFT'.
Can you help me to learn more?
(Other definitions for overdraft that I've seen before include "For advert, allowed to exceed amount lodged in bank" , "Bank's debt facility" , "Red balance" , "money that must be repaid" , "financial licence" .)