The most common and most likely answer to this crossword clue is the 5 plus 8 letter word FIRST MINISTER
I've seen this crossword clue in The Times
F
I
R
S
T
M
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N
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S
T
E
R
More clues leading to the result FIRST MINISTER
Buddy explains!

The straight part
We know that "leader" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case leader = first minister.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.