The most common and most likely answer to this crossword clue is the 4 letter word MALM
M
A
L
M
More clues leading to the result MALM
Buddy explains!

The straight part
We know that "Earthy stuff" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case earthy stuff = malm.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MALM mean?
MALM noun- A soft, crumbly, chalky, grayish limestone.
- An artificial mixture or chalk, clay, and sand, from which bricks are made. The resulting bricks have a light brown or yellowish color.