Unfocused, having no fun in old capital (6)

I believe the answer is:

escudo

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

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for escudo that I've seen before include "US code, oddly, used in Portugal until recently" , "Old Portuguese currency" , "Former currency of Portugal" , "Unit of Portuguese money" , "money in past" .)

I've seen this clue in The Irish Times.

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