The most common and most likely answer to this crossword clue is the 5 plus 10 letter word FIRST LIEUTENANT
I've seen this crossword clue in The Daily Telegraph
F
I
R
S
T
L
I
E
U
T
E
N
A
N
T
Buddy explains!
The straight part
We know that "Ship’s officer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ship’s officer = first lieutenant.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.