Crossword Genius

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

Ross

I believe the answer is:

scum

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'top' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for scum that I've seen before include "Floating impurities or bad people" , "Impurities floating on water, or despicable people" , "Matter floating on 9 ac" , "Dirty surface of a liquid" , "Dirt on liquid surface" .)

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