Goo, muck (6)

I believe the answer is:

sludge

This clue is a double definition.

'goo' is the first definition.
(synonyms)

'muck' is the second definition.
(synonyms)

(Other definitions for sludge that I've seen before include "'Muck, slime (6)'" , "Thick greasy mud" , "Ooze" , "Mire - sewage" , "Muddy sediment" .)

I've seen this clue in the Sydney Morning Herald.

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