Cruet liquid (3)
I believe the answer is:
oil
'cruet liquid' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oil that I've seen before include "SECOND PART OF 23" , "Grease" , "Thick liquid that doesn't dissolve in water" , "Black gold" , "Fuel, lubricant" .)