Decisive point (4)
I believe the answer is:
crux
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">decisive point</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for crux that I've seen before include "Key point" , "Main point (of the matter)" , "Most important point at issue" , "puzzle" , "Decisive, baffling, point" .)