Black gold (3)
I believe the answer is:
oil
'black gold' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oil that I've seen before include "Thick liquid that doesn't dissolve in water" , "Lubricant or petroleum" , "Type of fuel" , "Liquid lubricant" , "Lubricating liquid" .)