Uncommon thing (6)
I believe the answer is:
rarity
'uncommon thing' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rarity that I've seen before include "Something seldom seen" , "Infrequent occurrence" , "could be something of value" , "'Something unusual, perhaps collectable (6)'" , "It occurs infrequently" .)