Begin to show jumpiness? (5)

I believe the answer is:

start

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'begin' is the definition.
(I know that begin can be written as start)

I cannot understand how the rest of the clue works.

(Other definitions for start that I've seen before include "get under way" , "Commencement" , "Jerk" , "Opening - jump" , "Beginning" .)

I've seen this clue in the Evening Standard.

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