Stiffen (6)
'stiffen' is the definition.
(starching is a kind of stiffening)
This is all the clue.
(Other definitions for starch that I've seen before include "Formality - makes things stiff" , "Carbohydrate that occurs in all green plants" , "Carbohydrate found in potatoes, wheat etc." , "Carbohydrate; stiffen" , "Stiffening carbohydrate" .)
'stiffen' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for harden that I've seen before include "Set; become tougher" , "Become firm or tough" , "Anneal - solidify" , "Become stronger or firmer" , "Temper, steel say" .)