Lubricant from boiler (3)
I believe the answer is:
oil
'lubricant' is the definition.
(I've seen this before)
'from boiler' is the wordplay.
'from' indicates a hidden word.
'OIL' is hidden in the letters of 'boiler'.
(Other definitions for oil that I've seen before include "Fuel, lubricant" , "valuable commodity" , "Fossil fuel" , "Greasy liquid" , "Valuable viscous liquid" .)