Utterly incompetent (9)

I believe the answer is:

voiceless

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'utterly incompetent' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I don't see?

This is all the clue.

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(Other definitions for voiceless that I've seen before include "Dumb" , "Mum" .)

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

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