Crossword Genius

Isn’t her gentlest characteristic material resistance? (7,8)

Ross

I believe the answer is:

tensile strength

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'isn't her gentlest characteristic material resistance?' is the definition.
I can't tell whether this defines the answer.

'isn't her gentlest characteristic material resistance?' is the wordplay.
'isn't her gentlest' can be anagrammed to 'TENSILE STRENGTH'.
But, I am unsure how this is indicated.

Can you help me to learn more?

(Another definition for tensile strength that I've seen is " resistance at the point of breaking".)

I've seen this clue in the Sydney Morning Herald.
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