The most common and most likely answer to this crossword clue is the 4 plus 6 letter word KING ALFRED
I've seen this crossword clue in The Guardian
K
I
N
G
A
L
F
R
E
D
Buddy explains!
The straight part
We know that "leader once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case leader once = king alfred.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.