It makes oil company start to leave unknown area (5)
I believe the answer is:
colza
'it makes oil' is the definition.
The definition suggests a singular noun which matches the answer.
'company start to leave unknown area' is the wordplay.
'company' becomes 'co' (abbreviation).
'start to' indicates taking the first letters.
'unknown' becomes 'z' (represents an unknown value in algebra).
'area' becomes 'a' (maths abbreviation).
The first letter of 'leave' is 'l'.
'co'+'l'+'z'+'a'='COLZA'
Can you help me to learn more?
(Another definition for colza that I've seen is " that produces oil".)