Speak ill of (6)
'speak ill of' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for malign that I've seen before include "Evil - cancerous" , "Wicked, evil" , "Evil, baleful" , "Bad-mouth" , "Evil in nature" .)
(Other definitions for denigrate that I've seen before include "Criticise, belittle" , "Defame" , "put down" , "Bad-mouth" , "Disparage" .)