Mexico always provides fuel (4)
I believe the answer is:
coal
'fuel' is the definition.
(I know that coal is a type of fossil fuel)
'mexico always provides' is the wordplay.
'provides' indicates the answer is hidden within the clue.
'COAL' is hidden within 'mexico always'.
(Other definitions for coal that I've seen before include "Three domestic fuels" , "Black mineral used for burning" , "Carbon fuel" , "Fossil fuel" , "Carbonized vegetable matter" .)