Fossil fuel in eyeliner, we're told (4)

I believe the answer is:

coal

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'fossil fuel' is the definition.
(coal is a kind of fossil fuel)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for coal that I've seen before include "Black mineral used for burning" , "slack" , "Fossil fuel" , "Solid fuel" , "Carbon fuel" .)

I've seen this clue in The Everyman puzzle in The Observer.

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