American girls reject scum (4)

I believe the answer is:

slag

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'scum' is the definition.
(slag is a kind of scum)

I don't understand the rest of the clue.

(Other definitions for slag that I've seen before include "Smelting or coal waste" , "Mound of waste matter from mine" , "Cinders could become this" , "Smelted-ore refuse" , "Scum on metal - dissolute woman" .)

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

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