Occasionally driver steals fuel (6)
I believe the answer is:
diesel
'fuel' is the definition.
(diesel fuel is a kind of fuel)
'occasionally driver steals' is the wordplay.
'occasionally' means one should take alternating letters.
'driver'+'steals'='driversteals'
The alternating letters of 'driversteals' are 'DIESEL'.
(Other definitions for diesel that I've seen before include "Rudolf __ , French-born German engine inventor, d.1913" , "Fuel used by heavy vehicles" , "Lorry fuel" , "Derv" , "Eponymous German engineer" .)