Element discovered in 1898 (6)
I believe the answer is:
radium
'element discovered in 1898' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for radium that I've seen before include "Radioactive metal" , "Element with symbol Ra" , "Chemical element, Ra" , "Radioactive metallic element found in pitchblende" , "Metallic element discovered by the Curies" .)