Milk-curdling agent (6)
I believe the answer is:
rennet
'milk-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 "Tenner (anag.); cheese ingredient" , "Substance used in cheese making" , "apple" , "Milk curdling agent" , "Cheese curdling agent" .)