Chemical element unknown: discounting lead, tin and carbon (4)
I believe the answer is:
zinc
'chemical element' is the definition.
(zinc is a kind of chemical element)
'unknown discounting lead tin and carbon' is the wordplay.
'unknown' becomes 'z' (algebra).
'discounting lead' means to remove the first letter (discounting the letter which leads the word).
'and' means one lot of letters go next to another.
'carbon' becomes 'c' ('C' can be a synonym of 'carbon').
'tin' with its first letter taken away is 'in'.
'z'+'in'+'c'='ZINC'
(Other definitions for zinc that I've seen before include "coat for protection" , "Silvery-white metal" , "Metal alloyed with copper to make brass" , "Metallic element present with copper in brass" , "Ingredient in brass" .)