Wonder when 2, all but its first and second failing (6)
I believe the answer is:
rarity
'wonder' is the definition.
(I've seen this before)
'2 all but its first and second failing' is the wordplay.
I cannot really see how this works, but
'its' could be 'it' (I've seen this in other clues) and 'it' is found in the answer.
'first' could be 'a' (first letter of the alphabet) and 'a' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'when' acts as a link.
Can you help me to learn more?
(Other definitions for rarity that I've seen before include "Unusual specimen" , "Uncommon item or event" , "Very unusual occurrence" , "Something unusual, perhaps collectible" , "Thinness" .)