Crossword Genius

When about fifty, she is unequalled (7)

Ross

I believe the answer is:

peeress

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'when about fifty' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for peeress that I've seen before include "Female House of Lords member" , "She is married to a duke, earl etc" , "Noblewoman" , "Woman from other place" , "Female noble" .)

I've seen this clue in The Irish News.
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