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