Metal cask to hold gent’s rum (8)
I believe the answer is:
tungsten
'metal' is the definition.
(tungsten is a kind of metal)
'cask to hold gent's rum' is the wordplay.
'cask' becomes 'tun' (tun is a kind of cask).
'to hold' is an insertion indicator.
'rum' indicates anagramming the letters (rum can mean strange or unusual).
'gents' with letters rearranged gives 'ngste'.
'tun' enclosing 'ngste' is 'TUNGSTEN'.
(Other definitions for tungsten that I've seen before include "Element used in steels" , "Metallic element, W" , "Light-filament element" , "Chemical element, symbol W" , "Snug tent (anag)" .)