Sort of painting that's kind of slick (3)
I believe the answer is:
oil
I believe this clue is a double definition.
'sort of painting' is the first definition.
'kind of slick' is the second definition.
(I've seen this in another clue)
'that's' is the link.
(Other definitions for oil that I've seen before include "Lubricant or petroleum" , "Black gold" , "SECOND PART OF 23" , "Viscous substance as lubricant" , "Slippery liquid" .)