Rotten to the core (2,4)
I believe the answer is:
no good
'rotten to the core' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for no good that I've seen before include "Futile; worthless" , "Hopeless" , "(Of a person) beneath contempt" , "Useless, not worth bothering about" , "worthless sort" .)