Nuclear device (7)
I believe the answer is:
reactor
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nuclear device</span>' is the definition.<br/ ><span class="explanation-format__explanation">(nuclear reactor)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for reactor that I've seen before include "Atomic pile" , "(Nuclear) powerplant" , "power supplier" , "Nuclear power source" , "Nuclear apparatus for producing energy" .)