W sees mystery sleuth shadowing gangsters (4,4)

I believe the answer is:

moby dick

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

(Other definitions for moby dick that I've seen before include "Book by Herman Melville" , "Beast in book" , "Herman Melville's great novel of the White Whale" , "work by American" .)

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

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