The most common and most likely answer to this crossword clue is the 6 letter word SOOTED
S
O
O
T
E
D
Buddy explains!
The straight part
We know that "covered in dirty marks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case covered in dirty marks = sooted.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 SOOTED mean?
SOOTED adjective- Stained or marked with soot