The most common and most likely answer to this crossword clue is the 2 plus 3 plus 4 letter word OF ALL ARMS
O
F
A
L
L
A
R
M
S
Buddy explains!
The straight part
We know that "Across the military board" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case across the military board = of all arms.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.