Supremacy (7)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">supremacy</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that supremacy can be written as mastery)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for mastery that I've seen before include "Comprehensive skill" , "Comprehensive knowledge (of a subject)" , "Supremacy" , "superior knowledge" , "Expert skill" .)