Birdsong, perhaps, in worst dungeon, by the sound of it? (4,6)

I believe the answer is:

best seller

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

(Other definitions for best seller that I've seen before include "What many people are buying" , "Book bought by many" , "Popular book" , "Book bought in large numbers" , "Blockbuster" .)

I've seen this clue in The Guardian.

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