The most common and most likely answer to this crossword clue is the 12 letter word COMPANIONWAY
I've seen this crossword clue in The Sunday Telegraph
C
O
M
P
A
N
I
O
N
W
A
Y
Buddy explains!
The straight part
We know that "Steps down from deck" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case steps down from deck = companionway.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 COMPANIONWAY mean?
COMPANIONWAY noun- A staircase or ladder from one deck to another on a ship