Only's partner (3)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">only's partner</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for one that I've seen before include "time" , "Smallest whole number" , "Single, unitary" , "Individual" , "Ace" .)