Bones with curves round area towards the top (5)

I believe the answer is:

sacra

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'bones' is the definition.
(sacrum is a kind of bone)

I cannot understand how the remainder of the clue works.

(Other definitions for sacra that I've seen before include "Triangular bones" , "Bones that form the human pelvic arch" .)

I've seen this clue in The Times.

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