The "one" in a one-two
I believe the answer is:
jab
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">the one in a one-two</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 jab that I've seen before include "A short stiff punch" , "Poke roughly or inject" , "Injection" , "Prod" , "Short straight punch" .)