Lowest sort of people but always found at the top (4)
I believe the answer is:
scum
'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" .)