Fuel found in a boiler (3)
I believe the answer is:
oil
'fuel' is the definition.
(oil is a type of fuel)
'in a boiler' is the wordplay.
'in' indicates a hidden word.
'OIL' is hidden within 'boiler'.
'found' is the link.
(Other definitions for oil that I've seen before include "Lubricant or petroleum" , "Slippery liquid" , "Liquid gold" , "Greasy liquid" , "What can be extracted" .)