the fifth element (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">the fifth element</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for boron that I've seen before include "element needed for steel" , "Metalloid element, B" , "Non-metallic element, symbol B" , "component of steel" , "Chemical element, symbol B" .)