The most common and most likely answer to this crossword clue is the 8 letter word HATCHWAY
I've seen this crossword clue in The Times
H
A
T
C
H
W
A
Y
Buddy explains!

The straight part
We know that "opening in deck" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case opening in deck = hatchway.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 HATCHWAY mean?
HATCHWAY noun- A means of passing through a wall or floor, having a hatch (especially on a ship); a doorway with a hatch rather than a door.