Greasy (4)
I believe the answer is:
oily
'greasy' is the definition.
(I know that greasy can be written as oily)
This is all the clue.
(Other definitions for oily that I've seen before include "Smooth; like lubricant" , "Smoothly ingratiating" , "Viscous" , "Covered in lubricant" , "Unctuous" .)