The most common and most likely answer to this crossword clue is the 12 letter word AIDE-DE-CAMP
I've seen this crossword clue in The Guardian
A
I
D
E
-
D
E
-
C
A
M
P
Buddy explains!

The straight part
We know that "adjutant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case adjutant = aide-de-camp.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 AIDE-DE-CAMP mean?
AIDE-DE-CAMP noun- A military officer who serves as an adjutant to a higher ranking officer, prince or other high political dignitary.