Cover with hot fat, ending on roast in essence (5)
I believe the answer is:
baste
'cover with hot fat' is the definition.
(as in basting meat when cooking)
'ending on roast in essence' is the wordplay.
'ending on' indicates one should take the final letters.
'in' means one lot of letters goes inside another.
'essence' becomes 'base'.
The final letter of 'roast' is 't'.
't' put within 'base' is 'BASTE'.
(Other definitions for baste that I've seen before include "Pour hot fat over (meat)" , "Sew" , "strike" , "During roasting, spoon liquid over" , "Pour hot cooking juices over" .)