One's taken by Mark, perhaps, to be a metalloid (8)
I believe the answer is:
antimony
'a metalloid' is the definition.
(antimony is a metalloid element)
'one's taken by mark perhaps' is the wordplay.
'one's' becomes 'im' ('one is' is a formal way of saying 'I am').
'taken by' means one lot of letters goes inside another.
'mark perhaps' becomes 'antony' (Roman politician Mark Antony).
'im' going within 'antony' is 'ANTIMONY'.
'to be' is the link.
(Other definitions for antimony that I've seen before include "My nation (anag) - metallic element" , "Bluish-white element, symbol Sb" , "Semimetallic element" , "Toxic metallic element used in semiconductors" , "Brittle element" .)