The most common and most likely answer to this crossword clue is the 9 letter word GRUBBIEST
I've seen this crossword clue in The Guardian
G
R
U
B
B
I
E
S
T
Buddy explains!
The straight part
We know that "Muckiest/Muckiest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case muckiest/muckiest = grubbiest.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 GRUBBIEST mean?
GRUBBIEST adjective- Dirty, unwashed, unclean.
- Having grubs in it.