The most common and most likely answer to this crossword clue is the 8 letter word MANTRAPS
M
A
N
T
R
A
P
S
Buddy explains!

The straight part
We know that "Extreme security measures" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extreme security measures = mantraps.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.
What does MANTRAPS mean?
MANTRAPS noun- A mechanical device for catching trespassers.
- A small space with two sets of interlocking doors, such that the first set of doors must close before the second set opens, used to restrict access.
- A woman who is dangerously seductive to men.