Swamp thing (4)
I believe the answer is:
peat
'swamp thing' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for peat that I've seen before include "Bog material" , "Saturated and compressed vegetable matter, used as fuel when dried out" , "Form of fuel" , "Partially carbonised vegetable matter saturated with water" , "Organic fuel" .)