Sets off (6)
I believe the answer is:
starts
'sets off' is the definition.
('start' can be a synonym of 'set off')
This is the entire clue.
(Other definitions for starts that I've seen before include "Begins to make sudden surprised movements" , "Begins with surprise reactions?" , "Begins or flinches" , "reacts in surprise" , "Begins - or jumps as with fright" .)