The most common and most likely answer to this crossword clue is the 7 letter word GUNROOM
I've seen this crossword clue in The Guardian
G
U
N
R
O
O
M
Buddy explains!
The straight part
We know that "officers’ mess once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case officers’ mess once = gunroom.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 GUNROOM mean?
GUNROOM noun- A room where guns are stored.
- Living quarters for junior officers and midshipmen on a warship (hence gunroom officers). In the past it was usually set in the forecastle.