Amylum (6)
I believe the answer is:
starch
'amylum' is the definition.
(I know that amylum can be written as starch)
This is the entire clue.
(Other definitions for starch that I've seen before include "Potato, rice constituent; a stiffener" , "Stiffness; carbohydrate" , "ceremony" , "Stuff that stiffens cloth" , "Stiffening carbohydrate" .)