The most common and most likely answer to this crossword clue is the 9 plus 1 letter word TATTOOERS T
T
A
T
T
O
O
E
R
S
T
Buddy explains!
The straight part
We know that "They mark" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they mark = tattooers t.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.