Bad to the core (6)
I believe the answer is:
rotten
'bad to the core' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rotten that I've seen before include "Decayed - to the core" , "'Decayed, stinking (6)'" , "Something is ... in the state of Denmark (Hamlet)" , "Corrupt; very bad" , "Decomposed" .)