Do such numbers relate to quality? (5)

I believe the answer is:

prime

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'to quality?' is the definition.
('prime' can be a synonym of 'quality')

I don't understand the rest of the clue.

(Other definitions for prime that I've seen before include "Monastic service; best quality" , "(Of number) divisible only by itself and one" , "Of first quality" , "Capital - of first importance" , "Prepare to fire - first" .)

I've seen this clue in the Evening Standard.

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