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At this age, a fat lady has a round figure (6)

Ross

I believe the answer is:

eighty

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'figure' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

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(Other definitions for eighty that I've seen before include "XXC" , "Number" , "Fourscore" , "Cardinal" , "Northern county (abbr.)" .)

I've seen this clue in the Evening Standard.
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