Metal cask German sent abroad (8)
I believe the answer is:
tungsten
'metal' is the definition.
(tungsten is a kind of metal)
'cask german sent abroad' is the wordplay.
'cask' becomes 'tun' (tun is a kind of cask).
'german' becomes 'g' (abbreviation).
'abroad' indicates an anagram.
'sent' is an anagram of 'sten'.
'tun'+'g'+'sten'='TUNGSTEN'
(Other definitions for tungsten that I've seen before include "Metallic element used for lamp filaments" , "Element used in steels" , "Hard malleable metal" , "Lamp filament metal" , "Rare metallic element" .)