Bad name for a soft earthy substance (3)
I believe the answer is:
mud
This clue could be a double definition.
'bad name' is the first definition.
(if someone's 'name is mud' they are a disgrace)
'soft earthy substance' is the second definition.
'mud' can be an answer for 'substance' (mud is a kind of substance). I'm not certain of the 'soft earthy' bit.
'for a' acts as a link.
Can you help me to learn more?
(Other definitions for mud that I've seen before include "soft soil" , "Wet soil" , "Clay" , "in your eye, it's an affable toast" , "Liquid earth" .)