Powerfulness or effectiveness (7)
I believe the answer is:
potency
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">powerfulness</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(potency is a kind of powerfulness)</span><br/ ><br/ >'<span class="explanation-format__original">effectiveness</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for potency that I've seen before include "Power; pet cony (anag.)" , "Strength - effectiveness" , "Might" , "authority" , "Capacity" .)