The most common and most likely answer to this crossword clue is the 4 plus 3 plus 7 letter word HELP ONE ANOTHER
I've seen this crossword clue in Financial Times
H
E
L
P
O
N
E
A
N
O
T
H
E
R
Buddy explains!
The straight part
We know that "Mutual Assistance Directive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mutual assistance directive = help one another.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.