Part of some petrol engines (4)
I believe the answer is:
role
'part' is the definition.
(eg a theatrical role)
'some petrol engines' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'ROLE' is hidden in the letters of 'petrol engines'.
'of' is the link.
(Other definitions for role that I've seen before include "Part (in drama)" , "Impersonation" , "Function; part" , "Part played (in life?)" , "A part of lore" .)