A nation religiously on guard, perhaps (5)

I believe the answer is:

swiss

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'a nation' is the definition.
(I know that Swiss is a type of nation)

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

Can you help me to learn more?

(Other definitions for swiss that I've seen before include "-- Cheese plant; -- roll" , "Gruyere nationality" , "From Zurich, say?" , "like Federer" , "Sort of roll, of papal Guard" .)

I've seen this clue in the Evening Standard.

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