The most common and most likely answer to this crossword clue is the 8 plus 7 letter word SAWN-OFF SHOTGUN
I've seen this crossword clue in The Times
S
A
W
N
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O
F
F
S
H
O
T
G
U
N
More clues leading to the result SAWN-OFF SHOTGUN
Buddy explains!
The straight part
We know that "It’s loud when it goes off" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s loud when it goes off = sawn-off shotgun.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.