It's needed for things to run smoothly (3)

I believe the answer is:

oil

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'run smoothly' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for oil that I've seen before include "valuable commodity" , "Liquid gold" , "SECOND PART OF 23" , "Fuel, lubricant" , "Fossil fuel" .)

I've seen this clue in the Evening Standard.

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