Fuel from boiler (3)
I believe the answer is:
oil
'fuel' is the definition.
(oil is a type of fuel)
'from boiler' is the wordplay.
'from' indicates a hidden word.
'OIL' is hidden within 'boiler'.
(Other definitions for oil that I've seen before include "SECOND PART OF 23" , "Grease" , "Greasy liquid" , "Liquid lubricant" , "Viscous substance as lubricant" .)