Rounded down? (7)

I believe the answer is:

concave

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'rounded down?' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I don't see?

This is the entire clue.

Can you help me to learn more?

(Other definitions for concave that I've seen before include "like a bowl" , "Curving inwards" , "Curved inwards" , "Arch" , "Bending inwards" .)

I've seen this clue in The Globe and Mail.

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