Oil or grease (9)
I believe the answer is:
lubricant
This clue is a double definition.
'oil' is the first definition.
(I've seen this before)
'grease' is the second definition.
(I've seen this before)
(Other definitions for lubricant that I've seen before include "Substance used to reduce friction" , "What might have you well oiled" , "Oil, for example" , "E.g. oil" , "it has a smoothing effect" .)