Part of scale that is least fragile (8)

I believe the answer is:

solidest

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'least fragile' is the definition.
I can't tell whether this definition defines the answer.

'part of scale that is' is the wordplay.
'part of scale' becomes 'sol' (musical note in the solfege scale).
'that is' becomes 'id est' ('that is' in Latin).
'sol'+'idest'='SOLIDEST'

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I've seen this clue in The Times.

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