No good - energy in fuel is set (7)
I believe the answer is:
congeal
'is set' is the definition.
(I've seen this in another clue)
'no good energy in fuel' is the wordplay.
'no good' becomes 'ng' (abbreviation).
'energy' becomes 'e' (physics symbol as in E=mco).
'in' means one lot of letters goes inside another.
'fuel' becomes 'coal' (coal is a kind of fuel).
'ng'+'e'='nge'
'nge' put within 'coal' is 'CONGEAL'.
(Other definitions for congeal that I've seen before include "Set - go clean (anag)" , "Coagulate" , "Cool and solidify" , "Jell" , "Become solid" .)