The most common and most likely answer to this crossword clue is the 9 plus 5 letter word EXCLUSION ORDER
I've seen this crossword clue in The Daily Telegraph
E
X
C
L
U
S
I
O
N
O
R
D
E
R
Buddy explains!
The straight part
We know that "anti-terrorism measure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case anti-terrorism measure = exclusion order.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.