The most common and most likely answer to this crossword clue is the 10 plus 1 letter word MIDSHIPMAN A
I've seen this crossword clue in The Times
M
I
D
S
H
I
P
M
A
N
A
Buddy explains!
The straight part
We know that "Naval officer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case naval officer = midshipman a.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.