They were fabulously sour to the fox (6)

I believe the answer is:

grapes

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'they' is the definition.
Both the answer and definition are plural nouns.
Maybe there's a link between them I don't understand?

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

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(Other definitions for grapes that I've seen before include "Fruit that grows in clusters" , "Vine fruits" , "See 10 Across" , "Vine fruit" , "They grow on vines" .)

I've seen this clue in the Evening Standard.

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