Unit of radioactivity (5)
I believe the answer is:
curie
'unit of radioactivity' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for curie that I've seen before include "Unit of radioactivity" , "measure of radioactivity" , "Marie ---, found radium" , "Marie --, discoverer of Radium" , "Marie --, French chemist" .)