Not just one (4)
I believe the answer is:
both
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not just one</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 both that I've seen before include "The one and the other" , "The two (things)" , "couple" , "Each of two (alternatives)" , "Two considered together" .)