Smooth film of oil (5)
I believe the answer is:
slick
I believe this is a double definition.
'smooth' is the first definition.
(slicking is a kind of smoothing)
'film of oil' is the second definition.
(as in an oil slick)
(Other definitions for slick that I've seen before include "Bedeck" , "it's very oily" , "Glib; oil spill" , "Unwanted floater" , "Glibly assured" .)