Part of a petrol engine (4)
I believe the answer is:
role
'part' is the definition.
(eg a theatrical role)
'of a petrol engine' is the wordplay.
'of' indicates a hidden word.
'ROLE' is hidden in the letters of 'petrol engine'.
(Other definitions for role that I've seen before include "Part in play, film etc" , "Impersonation" , "Actor's part" , "Stage part" , "Part played (in life?)" .)