Fades, the Spanish fuel (6)
I believe the answer is:
diesel
'fuel' is the definition.
(diesel fuel is a kind of fuel)
'fades the spanish' is the wordplay.
'fades' becomes 'dies' (I've seen this in another clue).
'the spanish' becomes 'el' ('the' in Spanish).
'dies'+'el'='DIESEL'
(Other definitions for diesel that I've seen before include "Fuel used by heavy vehicles" , "Type of engine" , "Vehicle fuel" , "See lid for the engine" , "Train perhaps" .)