Weakened through heat? (9)

I believe the answer is:

qualified

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'weakened' is the definition.
(qualify can mean to weaken or make conditional)

I cannot understand how the rest of the clue works.

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(Other definitions for qualified that I've seen before include "Professional" , "Passed exams - with reservations" , "Having credentials" , "Provided with the attributes necessary to do a job" , "guarded" .)

I've seen this clue in The Times.

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