Crude oil initially featuring in vehicle's energy (6)
I believe the answer is:
coarse
'crude' is the definition.
(synonyms)
'oil initially featuring in vehicle's energy' is the wordplay.
'initially' says to take the initial letters.
'featuring in' is an insertion indicator.
'vehicle's' becomes 'car's' (car is a kind of vehicle).
'energy' becomes 'e' (physics symbol as in E=mc2).
The initial letter of 'oil' is 'o'.
'o' placed inside 'cars' is 'coars'.
'coars'+'e'='COARSE'
(Other definitions for coarse that I've seen before include "Boorish" , "Rough or rude" , "Rough, uncouth" , "Unrefined; rough" , "Inferior" .)