Begin to be jumpy (5)
I believe the answer is:
start
'begin' is the definition.
('start' can be a synonym of 'begin')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for start that I've seen before include "Set in motion" , "Take the first step" , "Jump suddenly" , "Onset" , "Commencement" .)