Pivotal; central (5)
I believe the answer is:
focal
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">pivotal</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">central</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ ></span>
(Other definitions for focal that I've seen before include "-- Length, -- plane; -- point" , "Relating to a centre of interest" , "Type of length or point" , "Of the point of convergence" , "- length characterises a lens" .)