Extremely small amount (4)
I believe the answer is:
iota
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">extremely small amount</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for iota that I've seen before include "Smidgen" , "Greek letter - minimal amount" , "A little piece" , "Ninthe letter of the Greek alphabet" , "very small part" .)