The most common and most likely answer to this crossword clue is the 6 letter word U-BOAT
I've seen this crossword clue in The Times
U
-
B
O
A
T
More clues leading to the result U-BOAT
- Sub turned about
- Vessel that was submersible but buoyant at regular intervals
- Sea vessel about to have components switched
- Submarine about to disintegrate
- Vessel restricted, contrarily packed with oxygen
- Note circling reptile is a threat, in the main
- Main threat stole lead from trader with uranium
- University club maintaining old submarine
- Craft using beads only, accessorising tank tops
- Uruguay originally stole Thailand’s torpedo carrier?
Buddy explains!
The straight part
We know that "Submarine" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case submarine = u-boat.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.