Engine driver so genial, surprisingly (8)
I believe the answer is:
gasoline
'engine driver' is the definition.
(the fuel that drives the engine)
'so genial surprisingly' is the wordplay.
'surprisingly' indicates anagramming the letters (a surprising ordering of the letters).
'so'+'genial'='sogenial'
'sogenial' anagrammed gives 'GASOLINE'.
(Other definitions for gasoline that I've seen before include "Fossil fuel" , "US petrol" , "US fuel" , "American petrol in full" , "N American petrol" .)