Stiffener (6)
I believe the answer is:
starch
'stiffener' is the definition.
(starch is used to make fabric stiff)
This is all the clue.
(Other definitions for starch that I've seen before include "Stiffener in the laundry" , "Stuff that stiffens, say collars" , "Formality - makes things stiff" , "Carbohydrate found in potatoes, wheat, and rice" , "Formality - give stiffness" .)