Part of the anatomy as the niacin produces muscular debility (10)

I believe the answer is:

myasthenia

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'muscular debility' is the definition.
The definition suggests a singular noun which matches the answer.

'part of the anatomy as the niacin' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'MYASTHENIA' is hidden within 'anatomy as the niacin'.

'produces' acts as a link.

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

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