Teacher in favour of oil company purchased by father (9)
I believe the answer is:
professor
'teacher' is the definition.
(I know that teacher can be written as professor)
'in favour of oil company purchased by father' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'oil company' becomes 'esso' (Esso is an oil company).
'purchased by' means one lot of letters goes inside another (inserted letters are acquired or purchased).
'father' becomes 'fr' (abbreviation as in a priest's title).
'esso' placed inside 'fr' is 'fessor'.
'pro'+'fessor'='PROFESSOR'
(Other definitions for professor that I've seen before include "Senior member of college faculty" , "University lecturer" , "Member of an academic faculty" , "One holds a chair" , "One affirming faith" .)