Crossword Genius

*Nonrenewable energy source . . . and the start of an eight-step word ladder (4)

Ross

I believe the answer is:

coal

(Other definitions for coal that I've seen before include "Char" , "This consists mainly of carbon" , "slack" , "Black mineral used for burning" , "Mined fuel" .)

I've seen this clue in The New York Times.
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