One had two banks purchase paper for the tellers (6)
I believe the answer is:
bireme
'one had two banks' is the definition.
(type of warship with two banks or oarsmen)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for bireme that I've seen before include "rowing boat" , "Type of ancient galley" , "old vessel?" , "Ancient galley with two banks of oars on each side" , "Ancient oared warship" .)