Make stiff (6)
I believe the answer is:
starch
'make stiff' is the definition.
(starching is a kind of making stiff)
This is all the clue.
(Other definitions for starch that I've seen before include "Formality - makes things stiff" , "Carbohydrate found in potatoes, wheat, and rice" , "foodstuff" , "Stiffness of manner" , "Fabric stiffener" .)