Diverting fuel will keep millions in cars, initially (7)
I believe the answer is:
comical
'diverting' is the definition.
(both can mean entertaining or amusing)
'fuel will keep millions in cars initially' is the wordplay.
'fuel' becomes 'coal' (coal is a kind of fuel).
'will keep' is an insertion indicator.
'initially' suggests taking the first letters.
The initial letters of 'millions in cars' is 'mic'.
'coal' enclosing 'mic' is 'COMICAL'.
(Other definitions for comical that I've seen before include "Rich" , "priceless" , "Witty" , "Humorous" , "Causing laughter" .)