Nuclear power source (7)
I think it could be either:
fast-breeder reactor
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nuclear power source</span>' is the definition.<br/ ><span class="explanation-format__explanation">(a nuclear reactor)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for reactor that I've seen before include "device that's energy-producing" , "chemical substance!" , "Producer of nuclear power" , "Nuclear power plant" , "Nuclear apparatus for producing energy" .)