The most common and most likely answer to this crossword clue is the 9 letter word LIGHTSHIP
I've seen this crossword clue in The Times
L
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P
More clues leading to the result LIGHTSHIP
- Pet food and joint provided in moored vessel
- Pilot's guide sets fire to joint
- Main beacon illuminates joint
- Vessel arrives on land with it
- . One anchored at sea sparks up joint
- Prepares to smoke joint, as a warning to seamen
- Sets fire to joint as warning for sailors
- Vessel resolving his plight
- His plight at sea with the beacon
- Navigational craft easily performed last of tasks with it
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 = lightship.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 LIGHTSHIP mean?
LIGHTSHIP noun- A vessel riding at anchor and displaying a light for the guidance of sailors, in a position where a fixed lighthouse structure would be impracticable.