The most common and most likely answer to this crossword clue is the 4 plus 2 plus 3 plus 1 plus 5 letter word THIS IS NOT A DRILL
I've seen this crossword clue in The Guardian
T
H
I
S
I
S
N
O
T
A
D
R
I
L
L
Buddy explains!
The straight part
We know that "It’s really happening!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s really happening! = this is not a drill.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.