One of two (6)
I believe the answer is:
either
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one of two</span>' is the definition.<br/ ><span class="explanation-format__explanation">(one of two options)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for either that I've seen before include "Word indicating the following alternatives" , "offered a choice" , "Each of two; moreover" , "Word introducing first of two 16s" , "No matter which (of two)" .)