Yeast (6)
I believe the answer is:
leaven
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">yeast</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 leaven that I've seen before include "It makes dough rise" , "Add yeast to" , "Substance for producing fermentation" , "work" , "Substance added to dough to make it rise" .)