Not one or the other (7)
I believe the answer is:
neither
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not one or the other</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for neither that I've seen before include "And not; nor yet" , "Therein lies not one or the other" , "double negative?" , "The rein (anag.)" , "Not one of two" .)