Fades, with the Spanish fuel (6)
I believe the answer is:
diesel
'fuel' is the definition.
(diesel fuel is a kind of fuel)
'fades with the spanish' is the wordplay.
'fades' becomes 'dies' (I've seen this in another clue).
'with' means one lot of letters go next to another.
'the spanish' becomes 'el' ('the' in Spanish).
'dies'+'el'='DIESEL'
(Other definitions for diesel that I've seen before include "See lid for the engine" , "Liquid fuel" , "Common fuel for heavy vehicles" , "Vehicle fuel" , "Derv" .)