The most common and most likely answer to this crossword clue is the 9 plus 5 letter word ANTARCTIC OCEAN
I've seen this crossword clue in The Guardian
A
N
T
A
R
C
T
I
C
O
C
E
A
N
More clues leading to the result ANTARCTIC OCEAN
Buddy explains!
The straight part
We know that "Main" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case main = antarctic ocean.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.