The most common and most likely answer to this crossword clue is the 8 letter word FINISHER
I've seen this crossword clue in The Times
F
I
N
I
S
H
E
R
Buddy explains!

The straight part
We know that "One crossing the line" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one crossing the line = finisher.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 FINISHER mean?
FINISHER noun- A person who finishes or completes something.
- A person who applies a finish to something, such as furniture.
- The person who applies the gilding and decoration in bookbinding.
- A construction machine used to smooth a newly constructed road surface.
- The blow that ends a fight; the knock-out blow.
- A finishing move.
- A player who shoots goals.
- A substitute player who plays at the end of the game.