Element unknown in Cape (4)
I believe the answer is:
zinc
'element' is the definition.
(zinc is a kind of element)
'unknown in cape' is the wordplay.
'unknown' becomes 'z' (algebra).
'cape' becomes 'c' (abbreviation used on maps).
'z'+'in'+'c'='ZINC'
(Other definitions for zinc that I've seen before include "Blue-white metallic element, atomic number 30" , "White metallic element usedin alloys" , "Constituent of brass" , "White metallic element used in alloys" , "it might be galvanising" .)