The most common and most likely answer to this crossword clue is the 13 letter word QUARTERDECKER
I've seen this crossword clue in The Guardian
Q
U
A
R
T
E
R
D
E
C
K
E
R
Buddy explains!
The straight part
We know that "a stickler for naval etiquette?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a stickler for naval etiquette? = quarterdecker.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.