Part inside petrol engine (4)
I believe the answer is:
role
'part' is the definition.
('role' can be a synonym of 'part')
'inside petrol engine' is the wordplay.
'inside' indicates a hidden word.
'ROLE' can be found hidden inside 'petrol engine'.
(Other definitions for role that I've seen before include "Stage part" , "Actor's part" , "Part, portrayal" , "Function in life" , "Character" .)