Worthless people, hundred in total (4)
I believe the answer is:
scum
'worthless people' is the definition.
(I've seen this before)
'hundred in total' is the wordplay.
'hundred' becomes 'c' (Roman numeral).
'in' indicates putting letters inside.
'total' becomes 'sum' ('sum' can be a synonym of 'total').
'c' placed within 'sum' is 'SCUM'.
(Other definitions for scum that I've seen before include "Worthless man" , "Dirt on a liquid's surface" , "lowlife" , "worst kind of people" , "Floating impurities or bad people" .)