Metal barrel fronting good gun (8)
I believe the answer is:
tungsten
'metal' is the definition.
(I know that tungsten is a type of metal)
'barrel fronting good gun' is the wordplay.
'barrel' becomes 'tun' (tun is a kind of barrel).
'fronting' says to take the initial letters (I've seen 'front' mean this).
'gun' becomes 'sten' (British sub-machine gun).
The initial letter of 'good' is 'g'.
'tun'+'g'+'sten'='TUNGSTEN'
(Other definitions for tungsten that I've seen before include "Greyish-white metal used in lamp filaments" , "Hard metal used in lamps etc" , "Chemical element, symbol W" , "Snug tent (anag)" , "Hard metal used in steels" .)