Weapon, presumably a good one (3)
I believe the answer is:
gun
'weapon' is the definition.
(gun is a kind of weapon)
'presumably a good one' is the wordplay.
I cannot quite see how this works, but
'good' could be 'g' (abbreviation) and 'g' is located in the answer.
'one' could be 'un' (northern English dialect form of 'one') and 'un' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for gun that I've seen before include "Platform from which to shoot" , "Revolver" , "It can be carried by 15 Ac" , "An offensive weapon" , "Hand weapon" .)