Jump at the sound of a pistol? (5)

I believe the answer is:

start

I'm a little stuck... Click here to teach me more about this clue!

'jump' is the definition.
(synonyms)

I can't explain the rest of the clue.

(Other definitions for start that I've seen before include "Origin" , "Commence" , "Commencement" , "open" , "Begin to recoil?" .)

Want a hint initially instead of a full solution? Install my app