The most common and most likely answer to this crossword clue is the 6 plus 6 letter word YELLOW JERSEY
I've seen this crossword clue in The Times
Y
E
L
L
O
W
J
E
R
S
E
Y
Buddy explains!
The straight part
We know that "leader's top" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case leader's top = yellow jersey.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.