Synthetic sweetener (9)
I believe the answer is:
saccharin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">synthetic sweetener</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 saccharin that I've seen before include "Synthetic sweetener" , "Sweet substance used as sugar substitute" , "Calorie-free sweetener" , "alternative sweetener" , "Very sweet compound" .)