The most common and most likely answer to this crossword clue is the 9 letter word DETONATES
I've seen this crossword clue in Financial Times
D
E
T
O
N
A
T
E
S
More clues leading to the result DETONATES
Buddy explains!

The straight part
We know that "Sets off" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sets off = detonates.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 DETONATES mean?
DETONATES verb- To explode; to blow up. Specifically, to combust supersonically via shock compression.
- To cause to explode.