Despicable people: hundred in total (4)
I believe the answer is:
scum
'despicable people' is the definition.
(I've seen this before)
'hundred in total' is the wordplay.
'hundred' becomes 'c' (Roman numeral).
'in' means one lot of letters goes inside another.
'total' becomes 'sum' (synonyms).
'c' inserted into 'sum' is 'SCUM'.
(Other definitions for scum that I've seen before include "Floating impurities or bad people" , "Impurities that rise to surface of liquid" , "Dirty foam" , "Offscourings" , "Impurities on surfaces of liquid" .)