6 down
Fossil
fuel
left
dons
suffering
agonies
(8)
Fossil fuel left dons suffering agonies (8)
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The definition is at the beginning of the clue.
How the clue works
Fossil fuel is the definition. Take L for "left" and pop it inside ("dons") an anagram of AGONIES, with "suffering" as the anagram indicator, giving GASO-L-INE.