The most common and most likely answer to this crossword clue is the 8 letter word MONORAIL
I've seen this crossword clue in The Times
M
O
N
O
R
A
I
L
Buddy explains!

The straight part
We know that "Line" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case line = monorail.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 MONORAIL mean?
MONORAIL noun- A railroad system where the trains run on one rail
- A train running on a single rail