Nuclear power generator (7)
I believe the answer is:
reactor
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nuclear power generator</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for reactor that I've seen before include "Nuclear energy generator" , "(Nuclear) powerplant" , "chemical substance!" , "source of power, perhaps" , "Nuclear power source" .)