Singleton (3)
I believe the answer is:
one
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">singleton</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 one that I've seen before include "Figure" , "Leading cardinal" , "doesn't count for much" , "United" , "Single and undivided" .)