The most common and most likely answer to this crossword clue is the 5 letter word WEALS
W
E
A
L
S
Buddy explains!
The straight part
We know that "Marks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case marks = weals.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 WEALS mean?
WEALS noun- A raised, longitudinal wound, usually purple, on the surface of flesh caused by a stroke of a rod or whip; a welt.