Crossword Genius

Boiler’s fuel (3)

Ross

I believe the answer is:

oil

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Mirror.
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