Dominant person (6)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dominant person</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for top dog that I've seen before include "Successful person - Crufts' winner?" , "Fat cat" , "One pre-eminent in their field" , "high in the sky?" , "big cheese?" .)