The most common and most likely answer to this crossword clue is the 5 plus 7 letter word GROUP CAPTAIN
I've seen this crossword clue in The Daily Telegraph
G
R
O
U
P
C
A
P
T
A
I
N
Buddy explains!
The straight part
We know that "military authority" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case military authority = group captain.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.