No good energy in fossil fuel, clot (7)
I believe the answer is:
congeal
'clot' is the definition.
(I've seen this before)
'no good energy in fossil fuel' is the wordplay.
'no good' becomes 'ng' (abbreviation).
'energy' becomes 'e' (physics symbol as in E=mc2).
'in' is an insertion indicator.
'fossil fuel' becomes 'coal' (coal is a kind of fossil fuel).
'ng'+'e'='nge'
'nge' going within 'coal' is 'CONGEAL'.
(Other definitions for congeal that I've seen before include "Become solid" , "Set - go clean (anag)" , "cake" , "Solidify by cooling" , "Cool and solidify" .)