Compact brownish soil deposit of partially decomposed organic matter used as a fuel (4)

I believe the answer is:

peat

(Other definitions for peat that I've seen before include "Fuel dug from bog" , "Partially carbonised vegetable matter used as a fuel when dried" , "Fuel from bogs" , "Organic fuel" , "Saturated and compressed vegetable matter, used as fuel when dried out" .)

I've seen this clue in The Mirror.

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