Atomic pile (7)
I believe the answer is:
reactor
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">atomic pile</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for reactor that I've seen before include "chemical substance!" , "nuclear facility" , "See 3 Down" , "nuclear vessel" , "(Nuclear) powerplant" .)