Radioactivity unit (5)
I believe the answer is:
curie
'radioactivity unit' is the definition.
(curie is a kind of radioactivity unit)
This is all the clue.
(Other definitions for curie that I've seen before include "Pioneer in radioactivity" , "Marie --, French chemist" , "Scientist" , "Marie ---, found radium" , "One of two Nobel prizewinners" .)