Dirty campaign stuff (3)
I believe the answer is:
'dirty campaign stuff' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mud that I've seen before include "Earth and water mixed" , "Soft wet earth" , "in your eye, it's an affable toast" , "damaging allegations" , "-, -, Glorious - (Flanders and Swann)" .)