Transmuting metal is primarily his concern (9)
I believe the answer is:
alchemist
'transmuting metal is primarily his concern' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'transmuting metal is primarily his concern' is the wordplay.
'transmuting' indicates anagramming the letters (I've seen 'transmuted' mean this).
'primarily' suggests taking the first letters.
The first letters of 'his concern' is 'hc'.
'metal'+'is'+'hc'='metalishc'
'metalishc' is an anagram of 'ALCHEMIST'.
Can you help me to learn more?
(Other definitions for alchemist that I've seen before include "Seeker after gold" , "St Michael (anag)" , "Medieval experimenter" , "... seeker after stone" , "magician" .)