The most common and most likely answer to this crossword clue is the 7 letter word DARKEST
I've seen this crossword clue in The Guardian
D
A
R
K
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 = darkest.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 DARKEST mean?
DARKEST adjective- Having an absolute or (more often) relative lack of light.
- (of colour) Dull or deeper in hue; not bright or light.
- Hidden, secret, obscure.
- Without moral or spiritual light; sinister, malign.
- Conducive to hopelessness; depressing or bleak.
- Lacking progress in science or the arts; said of a time period.
- With emphasis placed on the unpleasant aspects of life; said of a work of fiction, a work of nonfiction presented in narrative form or a portion of either.