Things struggling with fossil fuel waste (5,4)

I believe the answer is:

night soil

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'waste' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe you can see an association between them that I can't see?

'things struggling with fossil fuel' is the wordplay.
'struggling' is an anagram indicator (the letters struggle or wriggle into a new form).
'with' is a charade indicator (letters next to each other).
'fossil fuel' becomes 'oil' (I've seen this before).
'things' with letters rearranged gives 'nights'.
'nights'+'oil'='NIGHT-SOIL'

Can you help me to learn more?

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