Wheat protein (6)
I believe the answer is:
gluten
'wheat protein' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gluten that I've seen before include "Protein in cereal grain" , "Substance in cereal grain" , "Protein in wheat grains" , "Sticky substance" , "Substance contained in many grains" .)