Former leading lady? (9,2)

I believe the answer is:

elizabeth ii

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This might be a double definition.

'former leading' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'lady?' is the second definition.
(I know that Elizabeth II is a monarch of England)

Can you help me to learn more?

I've seen this clue in The Telegraph.

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