At this age, a fat lady has a round figure (6)
I believe the answer is:
eighty
'figure' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for eighty that I've seen before include "XXC" , "Number" , "Fourscore" , "Cardinal" , "Northern county (abbr.)" .)