The lowest sort of people but always found at the top (4)

I believe the answer is:

scum

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'the lowest' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for scum that I've seen before include "Floating dirt on surface" , "Impurities on surface of liquid, or rotten people" , "Impurities layer" , "Impurities that rise to surface of liquid" , "rabble" .)

I've seen this clue in The Guardian and The Globe and Mail.

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