Favourite job? Er ... producing oil (9)
I believe the answer is:
petroleum
'oil' is the definition.
(petroleum is a kind of oil)
'favourite job? er' is the wordplay.
'favourite' becomes 'pet' (synonyms).
'job?' becomes 'role' (a job is a role or position).
'er' becomes 'um' (both are expressions of uncertainty).
'pet'+'role'+'um'='PETROLEUM'
'producing' acts as a link.
(Other definitions for petroleum that I've seen before include "Fuel oil" , "Hydrocarbon oil" , "Source of fuel" , "Crude oil" , "Fossil fuel" .)