The most common and most likely answer to this crossword clue is the 7 letter word MANURES
I've seen this crossword clue in The Daily Telegraph
M
A
N
U
R
E
S
More clues leading to the result MANURES
Buddy explains!

The straight part
We know that "gets covered in muck" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gets covered in muck = manures.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 MANURES mean?
MANURES verb- To cultivate by manual labor; to till; hence, to develop by culture.
- To apply manure (as fertilizer or soil improver).