Substance in wheat (6)
I believe the answer is:
gluten
'substance in wheat' is the definition.
(I've seen this before)
This is all the clue.
(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" .)