They provide gold outside, having universal old currency (8)

I believe the answer is:

guilders

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'they' is the definition.
Both the definition and answer are plural nouns.
Maybe they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for guilders that I've seen before include "old coins" , "Units of money in Netherlands" .)

I've seen this clue in The Daily Express.

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