Jump the gun? (5)

I believe the answer is:

start

This clue is a double definition.

'jump' is the first definition.
('start' can be a synonym of 'jump')

'the gun?' is the second definition.
(thesaurus)

(Other definitions for start that I've seen before include "Begin with sudden movement of surprise" , "get under way" , "Begin - opening move" , "Jump suddenly" , "Set in motion" .)

I've seen this clue in the Evening Standard.

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