A weapon, and a good one (3)
I believe the answer is:
gun
'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.
(Other definitions for gun that I've seen before include "Revolver" , "Firearm" , "Platform from which to shoot" , "Arm" , "It can be carried by 15 Ac" .)