Assail, as a reputation (6)
I believe the answer is:
impugn
'assail as a reputation' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for impugn that I've seen before include "Blacken the name of" , "Challenge (e.g. honesty)" , "Nip mug badly and call in question" , "Express doubts about truth or honesty of, say motives" , "Attack verbally" .)