They guard treasures, yet possess no gems (6)

I believe the answer is:

gnomes

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'they' is the definition.
Both the answer and definition are plural nouns.
Perhaps you can see an association between them that I can't see?

'guard treasures yet possess no gems' is the wordplay.
'no gems' can be anagrammed to 'GNOMES'.
Though, I'm unsure how this is indicated.

Can you help me to learn more?

(Other definitions for gnomes that I've seen before include "Ornamental garden figures" , "Legendary little figures, in the garden perhaps" , "Legendary creatures" , "Garden dwarfs from Zurich?" , "Zurich goblins" .)

I've seen this clue in the Evening Standard.

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