Substance used for setting jams and jellies (6)
I believe the answer is:
pectin
'substance used for setting jams and jellies' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pectin that I've seen before include "Setting agent needed in jams" , "It thickens jam" , "Jam setter" , "Component of jam" , "Ingredient for setting jam" .)