Metal humidor contrivance (7)
I believe the answer is:
rhodium
'metal' is the definition.
(I know that rhodium is a type of Transition element)
'humidor contrivance' is the wordplay.
'contrivance' indicates anagramming the letters.
'humidor' with letters rearranged gives 'RHODIUM'.
(Other definitions for rhodium that I've seen before include "Hard metallic element" , "Chemical element, symbol Rh" , "Hard metal" , "Metallic element, atomic no 45" .)