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