Dirty political tactic (5)
I believe the answer is:
smear
'dirty political tactic' 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 "Slur - may be studied microscopically" , "False report" , "material for examination that is on the slide" , "Dirty mark; aspersion" , "Blot or daub" .)