Source of power (7,6)
(Other definitions for nuclear energy that I've seen before include "Elementary source of heat and electricity" , "Result of fission or fusion - near Uncle Grey (anag)" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">source of power</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for steam that I've seen before include "Kettle emission" , "Driving force" , "Vapour from water at boiling point" , "Vaporous exhalation" , "Vapour; energy" .)