Lubricant from the boiler (3)
I believe the answer is:
oil
'lubricant' is the definition.
(I've seen this before)
'from the boiler' is the wordplay.
'from' says the answer is hidden in the clue.
'OIL' is hidden in the letters of 'boiler'.
(Other definitions for oil that I've seen before include "Cooking agent" , "Grease" , "Lubricate; fuel" , "Fuel, lubricant" , "Viscous substance for lubrication" .)