The most common and most likely answer to this crossword clue is the 6 plus 6 letter word ARCTIC CIRCLE
I've seen this crossword clue in Financial Times
A
R
C
T
I
C
C
I
R
C
L
E
Buddy explains!
The straight part
We know that "a line on some /map/s" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a line on some /map/s = arctic circle.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.