The most common and most likely answer to this crossword clue is the 11 letter word PRE-EMINENT
I've seen this crossword clue in The Guardian
P
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E
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M
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More clues leading to the result PRE-EMINENT
Buddy explains!
The straight part
We know that "first" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case first = pre-eminent.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 PRE-EMINENT mean?
PRE-EMINENT adjective- Exceeding others in quality or rank; of outstanding excellence, extremely notable or important.