The most common and most likely answer to this crossword clue is the 8 letter word QUAINTER
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The straight part
We know that "Curiouser and curiouser" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case curiouser and curiouser = quainter.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does QUAINTER mean?
QUAINTER adjective- Of a person: cunning, crafty.
- Cleverly made; artfully contrived.
- Strange or odd; unusual.
- Overly discriminating or needlessly meticulous; fastidious; prim.
- Pleasingly unusual; especially, having old-fashioned charm.