Vilify (6)
(Other definitions for defame that I've seen before include "Damage (someone's) reputation" , "Calumniate, slander" , "Malign" , "'Denigrate, slander (6)'" , "Damage the reputation (of someone)" .)
'vilify' is the definition.
(I know that vilify can be written as revile)
This is the entire clue.
(Other definitions for revile that I've seen before include "Upbraid, berate" , "Vilify" , "Criticise in abusive language" , "Abusively criticise" , "Run down" .)