Substance in wheat (6)
I believe the answer is:
gluten
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">substance in wheat</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 gluten that I've seen before include "Protein in cereal grain" , "Substance contained in many grains" , "Protein source" , "Sticky protein present in cereal grains" , "Gluey protein substance in wheat" .)