Element unknown in carbon (4)
I believe the answer is:
zinc
'element' is the definition.
(zinc is a kind of element)
'unknown in carbon' is the wordplay.
'unknown' becomes 'z' (algebra).
'carbon' becomes 'c' (C is the chemical symbol for carbon).
'z'+'in'+'c'='ZINC'
(Other definitions for zinc that I've seen before include "Metal alloyed with copper to make brass" , "Constituent of brass" , "Ingredient in brass" , "Bluish-white metal" , "Bluish-white lustrous metallic element" .)