Stiffened or not! (5)

I believe the answer is:

boned

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'stiffened' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?

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

Can you help me to learn more?

(Another definition for boned that I've seen is " Filleted".)

I've seen this clue in the Evening Standard.

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