Spooner reckons a crustacean has some bone (6)
I believe the answer is:
sacrum
'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" .)