Stiffening agent (6)
I believe the answer is:
starch
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">stiffening agent</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)