Crossword Genius

Mature as a sphere? (7)

Ross

I believe the answer is:

rounded

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'mature' is the definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for rounded that I've seen before include "Plump" , "Complete and balanced; made curved" , "full" , "(Of the lips) pursed" , "Well-developed; curved" .)

I've seen this clue in The Mirror.
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