Speak badly of (7)
I believe the answer is:
traduce
'speak badly of' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for traduce that I've seen before include "Speak ill of" , "Defame, slander" , "Malign -- rude act (anag)" , "'Malign, badmouth (7)'" , "Misrepresent in unfavourable way" .)