Slanderous (10)
I believe the answer is:
defamatory
'slanderous' is the definition.
(I know that slanderous can be written as defamatory)
This is all the clue.
(Other definitions for defamatory that I've seen before include "Calumnious" , "Libellous" , "slanderous" , "abusive" , "Damaging to good reputation" .)