The most common and most likely answer to this crossword clue is the 8 letter word PALMIEST
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Buddy explains!
The straight part
We know that "most prosperous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most prosperous = palmiest.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 PALMIEST mean?
PALMIEST adjective- Made out of palm leaves or palm sap.
- Of, related to, or abounding in palm trees.
- Prosperous, flourishing, booming or thriving.