The most common and most likely answer to this crossword clue is the 5 letter word RO-RO
I've seen this crossword clue in Independent
R
O
-
R
O
Buddy explains!
The straight part
We know that "Connection out of Dover or Ostend" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case connection out of dover or ostend = ro-ro.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 RO-RO mean?
RO-RO noun- An individual vehicle of this kind.
- A form of marine vessel which allows wheeled vehicles to be driven on and off.