Blend of fuel key to energy (8)
I believe the answer is:
coalesce
'blend' is the definition.
(I've seen this before)
'fuel key to energy' is the wordplay.
'fuel' becomes 'coal' (coal is a kind of fuel).
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'to energy' becomes 'e' (physics symbol as in E=mc2).
'coal'+'esc'+'e'='COALESCE'
'of' is the link.
(Other definitions for coalesce that I've seen before include "To bring together" , "Unite" , "Merge into one" , "Come together in one mass" , "Make one whole" .)