Just a single (3)
I believe the answer is:
one
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">just a single</span>' is the definition.<br/ ><span class="explanation-format__explanation">(only one)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for one that I've seen before include "Single, unitary" , "1" , "This is singular" , "If upper-class, perhaps I" , "Unity" .)