Advanced years, when Thatcher was in her prime? (8)

I believe the answer is:

eighties

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'advanced years' is the definition.
Although both the answer and definition are plural nouns, I can't understand how they can define each other.

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

Can you help me to learn more?

(Other definitions for eighties that I've seen before include "Thatcher's PM decade" , "Period" , "Eighty to eighty nine" , "some years" .)

I've seen this clue in The Guardian.

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