Metal nuts get replaced by new (8)
I believe the answer is:
tungsten
'metal' is the definition.
(tungsten is a kind of metal)
'nuts get replaced by new' is the wordplay.
'replaced' indicates an anagram.
'by' says to put letters next to each other.
'new' becomes 'n'.
'nuts'+'get'='nutsget'
'nutsget' is an anagram of 'tungste'.
'tungste'+'n'='TUNGSTEN'
(Other definitions for tungsten that I've seen before include "Steel-grey metal" , "Wolfram" , "Chemical element, symbol W" , "Hard metal used in light bulb filaments" , "Hard malleable metal" .)