Crossword Genius

Coal set burning behind girl's back (7)

Ross

I believe the answer is:

lignite

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'coal' is the definition.
(I know that lignite is a type of coal)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for lignite that I've seen before include "Soft brownish coal" , "rock" , "it'll burn" , "Brown coal" , "Fuel" .)

I've seen this clue in The Daily Express.
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