Petroleum (3)
I believe the answer is:
oil
'petroleum' is the definition.
(I know that petroleum can be written as oil)
This is all the clue.
(Other definitions for oil that I've seen before include "Thick liquid that doesn't dissolve in water" , "SECOND PART OF 23" , "what can be seen on iceberg" , "Anoint" , "eg Olive" .)