Plant fungus (4)
I believe the answer is:
smut
'plant fungus' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for smut that I've seen before include "Dirt or immoral talk" , "Dirty stuff" , "Dirt or obscenity" , "Soot flake or sexual innuendo" , "Smudge of soot" .)