Entire mandible was broken? That can only be guessed (14)

I believe the answer is:

indeterminable

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

'entire mandible was broken? that can only be guessed' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?

'entire mandible was broken? that can only be guessed' is the wordplay.
'entire mandible' anagrams to 'INDETERMINABLE'.
But, I am not clear how this is indicated.

Can you help me to learn more?

(Another definition for indeterminable that I've seen is " can't be settled".)

I've seen this clue in The Daily Express.

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