Duck I left because it’s a bit greasy (3)
I believe the answer is:
Here is my best explanation:
'because it's a bit greasy' is the definition.
I can't tell whether this definition defines the answer.
'duck i left' is the subsidiary indication.
'duck' becomes 'o'.
'left' becomes 'l' (common abbreviation).
(Other definitions for oil that I've seen before include "Fuel, lubricant", "Viscous liquid", "Viscous substance for lubrication", "What can be extracted", "Viscous substance as lubricant", "Cooking agent", "Valuable viscous liquid", "Lubricant or petroleum", "Lubricating liquid" and "Lubricate; fuel".)