Some butter starts to go hard, even emulsify (4)
I believe the answer is:
ghee
'some butter' is the definition.
(ghee is a kind of butter)
'starts to go hard even emulsify' is the wordplay.
'starts to' suggests taking the first letters.
The first letters of 'go hard even emulsify' is 'GHEE'.
(Other definitions for ghee that I've seen before include "clarified product" , "Clarified butter sauce" , "Clarified butter used in Indian cookery" , "Clarified butter used in Indian cooking" , "Use it to cook" .)