Petroleum (3)
I believe the answer is:
oil
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">petroleum</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that petroleum can be written as oil)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)