Relating to atomic energy (7)
I believe the answer is:
nuclear
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">relating to atomic energy</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 nuclear that I've seen before include "Such a bomb" , "Relating to part of atom" , "winter, sort of" , "Clue ran oddly to atomic" , "source of energy" .)