To defame hard worker is evil (9)
I believe the answer is:
malignant
'is evil' is the definition.
(thesaurus)
'to defame hard worker' is the wordplay.
'to defame' becomes 'malign' (maligning is a kind of defaming).
'hard worker' becomes 'ant'.
'malign'+'ant'='MALIGNANT'
(Other definitions for malignant that I've seen before include "Injurious" , "Feeling or showing great ill will, cancerous" , "Bent on harming" , "Very bad" , "Evil, malevolent" .)