The worst part is on top (4)
I believe the answer is:
scum
This might be a double definition.
'the worst part' is the first definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'on top' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'is' acts as a link.
Can you help me to learn more?
(Other definitions for scum that I've seen before include "Floating rubbish" , "rabble" , "to be seen on top" , "Floating impurity layer" , "Floating impurities or bad people" .)