Stiffening agent (6)
I believe the answer is:
'stiffening agent' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for starch that I've seen before include "Stuff that stiffens cloth" , "Stiffener in the laundry" , "Carbohydrate found in potatoes, wheat, and rice" , "Charts (anag) - formality" , "Fabric stiffener" .)