Conks out, 'empty' sign, learner driver wanting fuel (6)
I believe the answer is:
diesel
'fuel' is the definition.
(I know that diesel is a type of petroleum products)
'conks out empty sign learner driver' is the wordplay.
'conks out' becomes 'dies' ('die' can be a synonym of 'conk out').
'empty sign' becomes 'E' (e.g. on a fuel guage).
'learner driver' becomes 'l' (learner drivers have L-plates on their cars).
'dies'+'e'+'l'='DIESEL'
'wanting' acts as a link.
(Other definitions for diesel that I've seen before include "Fuel used by heavy vehicles" , "Common fuel for heavy vehicles" , "See lid for the engine" , "Derv" , "Vehicle fuel" .)