Vitality, vigour (6)
I believe the answer is:
energy
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">vitality</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that vitality can be written as energy)</span><br/ ><br/ >'<span class="explanation-format__original">vigour</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that vigour can be written as energy)</span><br/ ><br/ ></span>
(Other definitions for energy that I've seen before include "Vitality, get-up-and-go" , "Vigour, spirit" , "Zeal" , "Vivacity" , "life" .)