One and the other
I believe the answer is:
both
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one and the other</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for both that I've seen before include "The two (things)" , "The one and the other" , "Each of two (alternatives)" , "couple" , "Two considered together" .)