The most common and most likely answer to this crossword clue is the 4 plus 5 letter word GRUB SCREW
I've seen this crossword clue in The Times
G
R
U
B
S
C
R
E
W
More clues leading to the result GRUB SCREW
Buddy explains!
The straight part
We know that "an aid to security" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an aid to security = grub screw.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.