Fossil fuel (4)
'fossil fuel' is the definition.
(gas is a fossil fuel)
This is all the clue.
(Other definitions for gas that I've seen before include "Empty talk (colloq.)" , "Hydrogen possibly" , "Hydrogen or methane, say" , "Oxygen maybe" , "See 2" .)
'fossil fuel' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for oil that I've seen before include "Fluid lubricant" , "eg Olive" , "Lubricant or petroleum" , "Lubricating liquid" , "Valuable viscous liquid" .)
(Other definitions for peat that I've seen before include "Partially carbonised vegetable matter used as a fuel when dried" , "Fuel from bogs" , "Fuel from marsh" , "Bog material" , "Natural solid fuel" .)
'fossil fuel' is the definition.
(coal is a kind of fossil fuel)
This is all the clue.
(Other definitions for coal that I've seen before include "Fossil fuel" , "Carbonaceous rock used as fuel" , "This consists mainly of carbon" , "slack" , "Solid fuel" .)