Decisive one (7)
I believe the answer is:
arbiter
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">decisive one</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for arbiter that I've seen before include "Decision maker" , "One decides in disputes" , "expert" , "Settler of disputes" , "Ref" .)