To defame is baleful (6)
I believe the answer is:
malign
I believe this clue is a double definition.
'to defame' is the first definition.
(I know that malign is a more specific form of the action asperse)
'baleful' is the second definition.
(thesaurus)
'is' is the link.
(Other definitions for malign that I've seen before include "Damaging - defame" , "Vilify, denigrate." , "Ill-disposed - defame" , "Speak ill of (someone)" , "Wicked, evil" .)