The most common and most likely answer to this crossword clue is the 9 letter word TREMBLERS
I've seen this crossword clue in The Guardian
T
R
E
M
B
L
E
R
S
Buddy explains!

The straight part
We know that "Nervous types/could set something off" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nervous types/could set something off = tremblers.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 TREMBLERS mean?
TREMBLERS noun- One who, or that which, trembles.
- Any of various New World passerine birds of the family Mimidae.
- The vibrating hammer, or spring contact piece of a hammer break, as of the electric ignition apparatus for an internal combustion engine.