Despicable fellow (3)
(Other definitions for cur that I've seen before include "Vicious dog" , "Mangy dog" , "Non-pedigree dog" , "Despicable person" , "A worthless dog or contemptible person" .)
'despicable fellow' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rotter that I've seen before include "rat" , "Pig" , "Absolute bounder" , "Scoundrel" , "Despicable person" .)