Set off a sudden involuntary movement (5)
I believe the answer is:
start
This might be a double definition.
'set off' is the first definition.
(synonyms)
'a sudden involuntary movement' is the second definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
Can you help me to learn more?
(Other definitions for start that I've seen before include "Outset" , "Set in motion" , "Commencement" , "Opening - jump" , "Initiate" .)