Heart of a nuclear power station (7)
I believe the answer is:
reactor
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">heart of a nuclear power station</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 power station core" , "chemical substance!" , "Nuclear power plant" , "one hosting moderator?" , "Nuclear installation" .)