The most common and most likely answer to this crossword clue is the 5 letter word VARIX
V
A
R
I
X
Buddy explains!

The straight part
We know that "vessel in a bad way" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vessel in a bad way = varix.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 VARIX mean?
VARIX noun- A varicose, i.e. swollen and knotted, vein
- In mollusks, a particular ridge on the shell, corresponding to a former position of the aperture.