The most common and most likely answer to this crossword clue is the 3 plus 4 letter word CAR BOMB
C
A
R
B
O
M
B
More clues leading to the result CAR BOMB
Buddy explains!

The straight part
We know that "terror threat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case terror threat = car bomb.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 CAR BOMB mean?
CAR BOMB verb- To attack with a car bomb