Cast aspersions on (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
impugn
(Other definitions for impugn that I've seen before include "Challenge (e.g. honesty)" , "Blacken the name of" , "Attack as false, challenge" , "Call into question; criticise" , "Try to discredit, express doubts about honesty" .)