Blended contents of solid fuel (3)
I believe the answer is:
oil
'fuel' is the definition.
(oil is a type of fuel)
'blended contents of solid' is the wordplay.
'blended' indicates anagramming the letters.
'contents of' means to look at the middle letters (the letters on the inside of the word).
The central letters of 'solid' are 'oli'.
'oli' is an anagram of 'OIL'.
(Other definitions for oil that I've seen before include "Viscous substance for lubrication" , "Valuable viscous liquid" , "Type of fuel" , "Crude" , "What can be extracted" .)