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