No description of dull pupils (5-4)

I believe the answer is:

sharp-eyed

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'no' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?

I don't understand how the rest of the clue works.

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(Another definition for sharp-eyed that I've seen is " Quick to notice".)

I've seen this clue in The Irish News.

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