A doctor is behind with a bill of exchange (5)
I believe the answer is:
draft
'a bill of exchange' is the definition.
(I know this)
'a doctor is behind' is the wordplay.
'a doctor' becomes 'Dr' (abbreviation).
'is behind' becomes 'aft' (aft means towards or at the end of a ship).
'dr'+'aft'='DRAFT'
'with' acts as a link.
(Other definitions for draft that I've seen before include "Send" , "Rough version" , "sketch, perhaps" , "Written plan" , "Bank payment order" .)