Equality with king usurping power is an uncommon thing (6)
I believe the answer is:
rarity
'uncommon thing' is the definition.
(I've seen this before)
'equality with king usurping power is an' is the wordplay.
I cannot really see how this works, but
'king' could be 'r' (abbreviation for rex, king in Latin) and 'r' is located in the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is found in the answer.
'power' could be 'art' (art is a kind of power) and 'art' is found within the remaining letters.
A single letter 'y' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rarity that I've seen before include "Something valued for its scarceness" , "Unusual item" , "Something not often seen" , "Uncommon item or event" , "It occurs infrequently" .)