Partially decayed vegetable matter (4)

I believe the answer is:

peat

(Other definitions for peat that I've seen before include "Vegetable deposit used as fuel and fertiliser" , "Organic fuel" , "Saturated and compressed vegetable matter, used as fuel when dried out" , "Fuel cut from bog" , "Bog vegetable matter, turf" .)

I've seen this clue in The Guardian.

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