The most common and most likely answer to this crossword clue is the 8 plus 4 letter word NAUTICAL MILE
I've seen this crossword clue in The Guardian
N
A
U
T
I
C
A
L
M
I
L
E
More clues leading to the result NAUTICAL MILE
Buddy explains!

The straight part
We know that "Main distance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case main distance = nautical mile.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.