Soft deposit possibly dug in part of Isleworth (6)

I believe the answer is:

sludge

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But I can't explain it at all! Could you help me understand?

(Other definitions for sludge that I've seen before include "Melting snow" , "Thick, soft, wet mud" , "Gloop, gunk, muck" , "Muddy sediment" , "Thick greasy mud" .)

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