The most common and most likely answer to this crossword clue is the 8 plus 7 letter word MILITARY ATTACHE
I've seen this crossword clue in The Guardian
M
I
L
I
T
A
R
Y
A
T
T
A
C
H
E
More clues leading to the result MILITARY ATTACHE
Buddy explains!
The straight part
We know that "Diplomatic officer/Diplomatic officer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case diplomatic officer/diplomatic officer = military attache.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.