West African country introducing unknown energy fuel mixture (7)
I believe the answer is:
benzine
'fuel mixture' is the definition.
(mixture of hydrocarbons from petroleum)
'west african country introducing unknown energy' is the wordplay.
'west african country' becomes 'benin' (country in West Africa).
'introducing' indicates putting letters inside (some letters introduced or inserted into others).
'unknown' becomes 'z' (algebra).
'energy' becomes 'e' (physics symbol as in E=mc2).
'benin' going around 'z' is 'benzin'.
'benzin'+'e'='BENZINE'
(Another definition for benzine that I've seen is " fuel".)