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 they are linked in a way I don't understand?

'things struggling with fossil fuel' is the wordplay.
'struggling' is an anagram indicator (the letters struggle or wriggle into a new form).
'with' means one lot of letters go next to another.
'fossil fuel' becomes 'oil' (I've seen this before).
'things' is an anagram of 'nights'.
'nights'+'oil'='NIGHT-SOIL'

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