Earth didn't start in a circular shape (5)

I believe the answer is:

round

I'm a little stuck... Click here to teach me more about this clue!

'shape' is the definition.
(I know that round is a more specific form of the action shape)

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

Can you help me to learn more?

(Other definitions for round that I've seen before include "Ignore fractions" , "Circular - stage" , "stout" , "with 6D [SPARE TYRE]" , "Number of drinks bought together" .)

I've seen this clue in The Daily Express.

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