Great working with new metal (6)
I believe the answer is:
argent
'metal' is the definition.
(thesaurus)
'great working with new' is the wordplay.
'working with' is an anagram indicator (working with the letters).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'great' with its letters anagrammed with 'n' can make 'ARGENT'.
(Other definitions for argent that I've seen before include "Silver on a coat of arms" , "(In heraldry) silver - garnet (anag)" , "Colour" , "White" , "Heraldic silver in garnet" .)