Diesel adulterated to include new oil source (7)
I believe the answer is:
linseed
'oil source' is the definition.
(I've seen this before)
'diesel adulterated to include new' is the wordplay.
'adulterated' is an anagram indicator (adulterated can mean spoiled).
'to include' is an insertion indicator.
'new' becomes 'n' (abbreviation as in New Testament NT).
'diesel' anagrammed gives 'liseed'.
'liseed' enclosing 'n' is 'LINSEED'.
(Other definitions for linseed that I've seen before include "Oil applied to cricket bats" , "Source of plant oil" , "Flax plant product providing oil" , "Enisled (anag.)" , "Part of flax used as oil" .)