The most common and most likely answer to this crossword clue is the 8 letter word LUSTIEST
I've seen this crossword clue in The Times
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The straight part
We know that "Most passionate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most passionate = lustiest.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 LUSTIEST mean?
LUSTIEST adjective- Exhibiting lust (in the obsolete sense meaning "vigor"); strong, healthy, robust; vigorous; full of sap or vitality.
- Hearty, merry, gleesome, enthusiastic, lively, stirring.
- Given to experiencing lust; enjoying physical sensations; lustful.
- Beautiful; handsome; pleasant.
- Of large size; big.