Alternative of two (5)
I believe the answer is:
other
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">alternative of two</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 other that I've seen before include "Alternate; additional" , "Different, disparate" , "Different, not the same one or this one" , "One may be significant" , "Very unusual" .)