The most common and most likely answer to this crossword clue is the 3 plus 3 letter word SEA DOG
I've seen this crossword clue in Financial Times
S
E
A
D
O
G
More clues leading to the result SEA DOG
- Old sailor aged, so wrinkly
- Sailor travels west, smuggling a daughter aboard
- Main setter possibly clues Ancient Mariner?
- Dosage ordered for a mariner
- Salt with main to follow
- Old sailor wants dosage adjusted
- Ship’s engineer initially having trouble with German sailor
- Boatman as main 22 down, say🙂
- Old hand travels northward to either side of land regularly
- Veteran on board turns up without notice
Buddy explains!

The straight part
We know that "Ancient Mariner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ancient mariner = sea dog.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 SEA DOG mean?
SEA DOG noun- A sailor accustomed to the sea.
- A pirate.
- A seal. (marine mammal)
- A charge resembling the talbot but with scales, webbed feet and a broad tail.
- A dogfish.
- An omen of bad luck.
- A white-topped wave in an estuary.