Boiler’s fuel (3)
I believe the answer is:
oil
'fuel' is the definition.
(oil is a type of fuel)
'boiler's' is the wordplay.
'OIL' is hidden in amongst the letters.
But, I am unsure how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for oil that I've seen before include "Thick liquid that doesn't dissolve in water" , "Black gold" , "What can be extracted" , "Cooking agent" , "Greasy liquid" .)