Double curve
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">double curve</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 ess that I've seen before include "Letter from Mid-West" , "Curvaceous letter" , "Alphabet letter" , "Nineteenth letter" , "top secret" .)