Nasty relative listened to row (6)
I believe the answer is:
malign
'nasty' is the definition.
(thesaurus)
'relative listened to row' is the wordplay.
I cannot really see how this works, but
'relative' could be 'ma' (ma is a kind of relative) and 'ma' is found in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for malign that I've seen before include "Speak bad of" , "Showing malice, say harmful things about" , "Evil in effect" , "run down" , "Wicked, evil" .)