Curdling agent (6)
I believe the answer is:
rennet
'curdling agent' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rennet that I've seen before include "Substance used in cheese making" , "cheesy setter?" , "Cheese-maker's substance" , "Cheese-making curdler" , "Cheese curdling agent" .)