Crossword Genius

Spooner reckons a crustacean has some bone (6)

Ross

I believe the answer is:

sacrum

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

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

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

Can you help me to learn more?

(Other definitions for sacrum that I've seen before include "Triangular bone forming back of pelvis" , "Bone in the lower back" , "Wedge-shaped bone forming the back part of the pelvis" , "Triangular bone that forms back of pelvis" .)

I've seen this clue in the Sydney Morning Herald.
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