Metal tin, a lump processed (8)
I believe the answer is:
platinum
'metal' is the definition.
(I know that platinum is a type of noble metal)
'tin a lump processed' is the wordplay.
'processed' is an anagram indicator.
'tin'+'a'+'lump'='tinalump'
'tinalump' anagrammed gives 'PLATINUM'.
(Other definitions for platinum that I've seen before include "Atomic number 78" , "12 down metal" , "Valuable substance" , "Heavy white metal" , "Precious metal, Pt" .)