Crossword Genius

One had two banks purchase paper for the tellers (6)

Ross

I believe the answer is:

bireme

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'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.

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(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" .)

I've seen this clue in The Times.
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