Relating to nuclear energy (6)
I believe the answer is:
atomic
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">relating to nuclear 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 atomic that I've seen before include "of bombs" , "Based on nuclear energy" , "Of very small particles" , "Using nuclear energy" , "Relating to basic particles" .)