Fuel source from bogs (4)
I believe the answer is:
peat
'fuel source from bogs' 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 "Fuel from bogs" , "Partially carbonised vegetable matter saturated with water" , "Partially carbonised vegetable matter used as a fuel when dried" , "Dried plant material used as fuel" , "Organic fuel" .)