In cooking, he employed clarified butter (4)
I believe the answer is:
ghee
'clarified butter' is the definition.
(ghee is a kind of clarified butter)
'in cooking he employed' is the wordplay.
'in' indicates the answer is hidden within the clue.
'GHEE' can be found hidden inside 'cooking he employed'.
(Other definitions for ghee that I've seen before include "Clarified butter used in Indian cookery" , "clarified product" , "Clarified butter used in Indian cooking" , "type of butter" , "Clarified butter sauce" .)