Crossword Genius

Bad sort of runner, but possibly starting a race! (3)

Ross

I believe the answer is:

gun

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

'race' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for gun that I've seen before include "Rev up" , "Shooter" , "Platform from which to shoot" , "Arm" , "What has barrel" .)

I've seen this clue in the Evening Standard.
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