A 'jump' one for the motorist? (5)

I believe the answer is:

start

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'a jump' is the definition.
(as in starting or jumping in fear)

I don't understand the remainder of the clue.

(Other definitions for start that I've seen before include "Begin with sudden movement of surprise" , "open" , "Outset" , "Begin; flinch" , "Initiate" .)

I've seen this clue in the Evening Standard.

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