A jot (4)
I believe the answer is:
iota
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a jot</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for iota that I've seen before include "Greek letter - small quantity" , "A little piece" , "letter from Greece" , "Trace" , "very small part" .)