Junket ingredient (6)
I believe the answer is:
rennet
'junket ingredient' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rennet that I've seen before include "substance used for curdling" , "Substance used to curdle milk for cheese-making" , "Cheese curdling agent" , ""This has turned the milk!"" , "Substance curdling milk in cheese-making" .)