Horrible people, hundred in total (4)
I believe the answer is:
scum
'horrible people hundred in total' is the definition.
I can't judge whether this definition defines the answer.
'hundred in total' is the wordplay.
'hundred' becomes 'c' (Roman numeral).
'in' indicates putting letters inside.
'total' becomes 'sum' (synonyms).
'c' inserted into 'sum' is 'SCUM'.
Can you help me to learn more?
(Other definitions for scum that I've seen before include "Impurities on liquid's surface" , "Refuse" , "Undesirable people" , "The lowest" , "Dregs (of society?)" .)