The most common and most likely answer to this crossword clue is the 5 letter word WARTS
I've seen this crossword clue in Financial Times
W
A
R
T
S
More clues leading to the result WARTS
Buddy explains!

The straight part
We know that "unsightly marks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unsightly marks = warts.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 WARTS mean?
WARTS noun- A type of deformed growth occurring on the skin caused by the human papillomavirus (HPV).
- Any similar growth occurring in plants or animals, such as the parotoid glands in the back of toads.
- Any of the prefixes used in Hungarian notation.