The most common and most likely answer to this crossword clue is the 5 letter word OILER
I've seen this crossword clue in The Guardian
O
I
L
E
R
More clues leading to the result OILER
Buddy explains!

The straight part
We know that "vessel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vessel = oiler.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 OILER mean?
OILER noun- One who or that which oils.
- An oil tanker.
- An oil well.
- A small (typically thumb-sized) metal container of oil, often containing an integral brush.
- An oilskin coat.