Coal set burning behind girl's back (7)
I believe the answer is:
lignite
'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" .)