The most common and most likely answer to this crossword clue is the 12 letter word WORLD-BEATER
I've seen this crossword clue in The Times
W
O
R
L
D
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B
E
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R
More clues leading to the result WORLD-BEATER
Buddy explains!
The straight part
We know that "someone widely seen as leader" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case someone widely seen as leader = world-beater.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.
Crossword Answer definitions
WORLD-BEATER noun- Someone or something superior to all others of its sort.