Element discovered by Marie and Pierre Curie (6)
I believe the answer is:
radium
'element discovered by marie and pierre curie' 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 "Element with symbol Ra" , "Curie-found element" , "Ra (atomic number 88)" , "Radioactive metallic element found in pitchblende" , "Chemical element, Ra" .)