Radioactive metal used as fuel in nuclear reactors (7)
I believe the answer is:
uranium
'radioactive metal used as fuel in nuclear reactors' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for uranium that I've seen before include "Radioactive element of atomic number 92" , "Mineral used in nuclear reactors" , "Chemical element, source of nuclear energy" , "Chemical element used to get nuclear energy" , "Radoactive element" .)