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.
Perhaps they are linked in a way I don't understand?

I don't understand the rest of the clue.

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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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