Despicable dope! (3-4)
I believe the answer is:
low-down
This clue is a double definition.
'despicable' is the first definition.
(thesaurus)
'dope' is the second definition.
(I know that dope can be written as low-down)
(Other definitions for low-down that I've seen before include "Inside information - mean (7;3-4)" , "Mean, unfair" , "Info (slang) - dishonourable" , "Inside story" , "Data - base" .)