Unusual thing (6)
(Other definitions for rarity that I've seen before include "Uncommon object" , "Something not often seen" , "Curio (1)" , "'Something unusual, perhaps collectable (6)'" , "Find" .)
'unusual thing' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oddity that I've seen before include "Curious thing" , "Quality of strangeness" , "Peculiarity" , "Unusual thing" , "Quirk" .)