As weapons go, a good one (3)

I believe the answer is:

gun

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'as weapons go a' is the definition.
I can't tell whether this definition defines the answer.

'good one' is the wordplay.
'good' becomes 'g' (abbreviation).
'one' becomes 'un' (northern English dialect form of 'one').
'g'+'un'='GUN'

Can you help me to learn more?

(Other definitions for gun that I've seen before include "Shooter" , "It can be carried by 15 Ac" , "What has barrel" , "Arm" , "Firearm" .)

I've seen this clue in the Evening Standard.

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