Amount owed concerning doctor at the back (9)
I believe the answer is:
overdraft
'amount owed' is the definition.
(amount owed to a bank)
'concerning doctor at the back' is the wordplay.
'concerning' becomes 'over' (both can mean relating to).
'doctor' becomes 'dr' (abbreviation).
'at the back' becomes 'aft' (aft means towards or at the end of a ship).
'over'+'dr'+'aft'='OVERDRAFT'
(Other definitions for overdraft that I've seen before include "Bank facility to go into the red" , "Bank lending arrangement" , "Bank account deficit" , "Potential problem with bank" , "Debit balance in bank account" .)