The most common and most likely answer to this crossword clue is the 13 plus 5 letter word NEIGHBOURHOOD WATCH
I've seen this crossword clue in The Times
N
E
I
G
H
B
O
U
R
H
O
O
D
W
A
T
C
H
Buddy explains!
The straight part
We know that "Security association" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case security association = neighbourhood watch.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.