Barrel with good gun metal (8)
I believe the answer is:
tungsten
'metal' is the definition.
(I know that tungsten is a type of metal)
'barrel with good gun' is the wordplay.
'barrel' becomes 'tun' (tun is a kind of barrel).
'with' says to put letters next to each other.
'good' becomes 'g' (abbreviation).
'gun' becomes 'sten' (British sub-machine gun).
'tun'+'g'+'sten'='TUNGSTEN'
(Other definitions for tungsten that I've seen before include "Hard metal used in lamps etc" , "Hard metal used in steels" , "Very hard metal, used in light bulbs" , "Lamp filament metal" , "Metallic element in a strangely snug tent" .)