Minute pun about a metal (9)
I believe the answer is:
neptunium
'a metal' is the definition.
(I know that neptunium is a type of metal)
'minute pun about' is the wordplay.
'about' indicates an anagram.
'minute'+'pun'='minutepun'
'minutepun' anagrammed gives 'NEPTUNIUM'.
(Other definitions for neptunium that I've seen before include "Element named after planet" , "Radioactive metal" , "scarce chemical" .)