Metal barrel by gents removed (8)
I believe the answer is:
tungsten
'metal' is the definition.
(tungsten is a kind of metal)
'barrel by gents removed' is the wordplay.
'barrel' becomes 'tun' (tun is a kind of barrel**).
'by' means one lot of letters go next to another.
'removed' is an anagram indicator (moved again).
'gents' anagrammed gives 'gsten'.
'tun'+'gsten'='TUNGSTEN'
(Other definitions for tungsten that I've seen before include "Hard metal used in light bulb filaments" , "Metallic element in a strangely snug tent" , "Element used in electric light filaments" , "Light-filament element" , "Rare metallic element" .)