Hydrocarbon present in petroleum (7)
I believe the answer is:
toluene
'hydrocarbon present in petroleum' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for toluene that I've seen before include "Compound" , "flammable liquid" , "Hydrocarbon present in petroleum" , "Constituent of petroleum" , "One lute (anag.); a solvent" .)