Crossword Genius

A weapon, and a good one (3)

Ross

I believe the answer is:

gun

Here is my best explanation:

'a weapon' is the definition.
(gun is a kind of weapon)

'a good one' is the wordplay.
'a good' becomes 'g' (abbreviation).
'one' becomes 'un' (I've seen this in other clues).
'g'+'un'='GUN'

'and' is the link.

This clue was last seen in Evening Standard