The most common and most likely answer to this crossword clue is the 10 letter word LONGITUDES
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result LONGITUDES
Buddy explains!
The straight part
We know that "Lines up the mariner’s chart?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lines up the mariner’s chart? = longitudes.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 LONGITUDES mean?
LONGITUDES noun- Angular distance measured west or east of the prime meridian.
- Any imaginary line perpendicular to the equator and part of a great circle passing through the North Pole and South Pole.
- Length.