It's not refined, but it could be slick (5,3)

I believe the answer is:

crude oil

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'be slick' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand the remainder of the clue.

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(Other definitions for crude oil that I've seen before include "that has boring producers?" , "liquid asset" , "Source of fuel" , "From which petrol is made" , "Unrefined petroleum" .)

I've seen this clue in The Independent.

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