Drag through the mud (5)
I believe the answer is:
smear
'drag through the mud' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for smear that I've seen before include "Slander - tissue sample used for testing" , "Slur - may be studied microscopically" , "nasty mark" , "Unwarranted accusation" , "Blot or daub" .)