Stiffener (6)
I believe the answer is:
starch
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">stiffener</span>' is the definition.<br/ ><span class="explanation-format__explanation">(starch is used to make fabric stiff)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)