Equality – with prince becoming king – it's seldom seen (6)

I believe the answer is:

rarity

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'it's seldom seen' is the definition.
(I've seen this in another clue)

I can't explain the remainder of the clue.

(Other definitions for rarity that I've seen before include "Something not often seen" , "could be something of value" , "Uncommon thing" , "Uncommon item or event" , "Curio (1)" .)

I've seen this clue in The Everyman puzzle in The Observer.

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