Vital point (4)
I believe the answer is:
crux
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">vital point</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 crux that I've seen before include "Decisive point of an issue" , "Key point" , "Essential point in the matter or problem" , "Point of difficulty" , "brain-teaser!" .)