Evidence treated as unassailable (5)

I believe the answer is:

proof

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'evidence' is the definition.
(evidence proves something)

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

(Other definitions for proof that I've seen before include "Strength of spirit" , "Clear evidence" , "Trial printing" , "mathematical work" , "Measure of the strength of alcohol" .)

I've seen this clue in The Globe and Mail.

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