Barrel -- good gun metal (8)
I believe the answer is:
tungsten
'metal' is the definition.
(tungsten is a kind of metal)
'barrel good gun' is the wordplay.
'barrel' becomes 'tun' (tun is a kind of barrel).
'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 "Light-filament element" , "Greyish-white metal used in lamp filaments" , "Element used in electric light filaments" , "Snug tent (anag)" , "Rare metallic element" .)