Something you won't come across very often (6)
I believe the answer is:
rarity
'you won't come across very often' is the definition.
I can't judge whether this defines the answer.
'something' is the wordplay.
I cannot quite understand how this works, but
'something' could be 'it' (I've seen this in other clues) and 'it' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rarity that I've seen before include "Uncommon item or event" , "Uncommon thing" , "Something not often seen" , "Find" , "Unusual thing" .)