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

I believe the answer is:

existed

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'no longer is' is the definition.
I can't tell whether this defines the answer.

'once the diet's worked' is the wordplay.
'once' becomes 'ex' (prefix meaning former or onetime).
'worked' is an anagram indicator.
'diets' anagrammed gives 'isted'.
'ex'+'isted'='EXISTED'

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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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