Muck turning putrid, mostly (4)
I believe the answer is:
dirt
'muck' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for dirt that I've seen before include "Obscenity" , "'Mud, dust or grime (4)'" , "Muck" , "Scandalous information" , "Grime - scandal" .)