Metallic element unknown in China’s capital (4)
I believe the answer is:
zinc
'metallic element' is the definition.
(zinc is a kind of metallic element)
'unknown in china's capital' is the wordplay.
'unknown' becomes 'z' (algebra).
'capital' indicates taking the first letters.
The initial letter of 'chinas' is 'c'.
'z'+'in'+'c'='ZINC'
(Other definitions for zinc that I've seen before include "Metal used for galvanising steel" , "Blusish-white lustrous metallic element" , "Blue-white metallic element, atomic number 30" , "Metal used in brass" , "Metallic element present with copper in brass" .)