What does the cryptic crossword clue "Wonder when 2, all but its first and second failing (6)" mean?

The definition is "Wonder". The wordplay uses an anagram. anagram of RTIARY = RARITY; definition: "Wonder" The answer is RARITY.

Wonder when 2, all but its first and second failing (6)

Guardian Cryptic #29239 | 20 Down | 2023-11-28 flower