Slick stuff (3)
I believe the answer is:
oil
'slick stuff' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oil that I've seen before include "Greasy liquid" , "Fuel, lubricant" , "Lubricating liquid" , "Valuable viscous liquid" , "Viscous substance as lubricant" .)