Royal charity starts losing it's unusualness (6)

I believe the answer is:

rarity

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'it's unusualness' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for rarity that I've seen before include "Something seldom seen" , "Collector's item" , "Find" , "'Something unusual, perhaps collectable (6)'" , "Something valued for its scarceness" .)

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

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