German currency between banks? (5)

I believe the answer is:

rhine

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'german' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for rhine that I've seen before include "water" , "River rising in Alps, with mouth at Rotterdam" , "River flowing from Switzerland, through Germany, to the North Sea" , "Great German river with Lorelei Rock" , "Ring river (Wagner)" .)

I've seen this clue in The Globe and Mail.

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