Filth from curmudgeon (3)
I believe the answer is:
mud
'filth' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'from curmudgeon' is the wordplay.
'from' says the answer is hidden in the clue.
'MUD' is hidden within 'curmudgeon'.
Can you help me to learn more?
(Other definitions for mud that I've seen before include "Here's . . . in your eye - it's a toast" , "Soft wet earth" , "soft soil" , "Mire" , "Wet sticky earth" .)