The most common and most likely answer to this crossword clue is the 4 letter word SLAG
I've seen this crossword clue in The Daily Telegraph
S
L
A
G
Buddy explains!
The straight part
We know that "Scum" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case scum = slag.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SLAG mean?
SLAG noun- Waste material from a coal mine
- Scum that forms on the surface of molten metal
- Impurities formed and separated out when a metal is smelted from ore; vitrified cinders
- Hard aggregate remaining as a residue from blast furnaces, sometimes used as a surfacing material
- Scoria associated with a volcano
- A coward
- (chiefly Cockney) a contemptible person, a scumbag
- A prostitute
- A woman (sometimes a man) who has loose morals relating to sex; a slut
- To produce slag
- To become slag; to agglomerate when heated below the fusion point
- To reduce to slag
- (sometimes with "off") to talk badly about; to malign or denigrate (someone)
- To spit