Organism causing fermentation (5)
I believe the answer is:
yeast
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">organism causing fermentation</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 yeast that I've seen before include "Fungus used to ferment sugar" , "Brewer's agent" , "Brewing agent" , "that baker needs" , "Rising agent" .)