Power, vigour (8)
I believe the answer is:
strength
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">power</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that strength is a type of power)</span><br/ ><br/ >'<span class="explanation-format__original">vigour</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for strength that I've seen before include "The longest one-syllable word in English - might" , "Brawn, muscle power." , "Force, might, potency" , "Force, firmness" , "Beneficial attribute" .)