Doctor back to get banker's order (5)
I believe the answer is:
draft
'banker's order' is the definition.
(a written order to pay a sum)
'doctor back' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'back' becomes 'aft' (associated in meaning).
'dr'+'aft'='DRAFT'
'to get' acts as a link.
(Other definitions for draft that I've seen before include "book's outline" , "conscript" , "form of conscription" , "Sketch - detachment of men" , "Provisional document" .)