The most common and most likely answer to this crossword clue is the 5 letter word RHUMB
R
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More clues leading to the result RHUMB
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 = rhumb.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 RHUMB mean?
RHUMB noun- A line which crosses successive meridians at a constant angle
- One of the 32 points of the compass (compass points)
- A unit of angular measure equal to 1/32 of a circle or 11.25°