The most common and most likely answer to this crossword clue is the 9 letter word FILTHIEST
I've seen this crossword clue in The Guardian
F
I
L
T
H
I
E
S
T
Buddy explains!
The straight part
We know that "most dirty" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most dirty = filthiest.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 FILTHIEST mean?
FILTHIEST adjective- Covered with filth; very dirty.
- Obscene or offensive.
- Very unpleasant or disagreeable.