Once the diet's worked, no longer is (7)

I believe the answer is:

existed

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'once' is the definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives sometimes mean the same thing.

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

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(Another definition for existed that I've seen is " Lived".)

I've seen this clue in the Evening Standard.

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