The most common and most likely answer to this crossword clue is the 9 plus 6 letter word COMPANION LADDER
I've seen this crossword clue in The Daily Telegraph
C
O
M
P
A
N
I
O
N
L
A
D
D
E
R
More clues leading to the result COMPANION LADDER
Buddy explains!
The straight part
We know that "one’s located between decks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one’s located between decks = companion ladder.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.