The most common and most likely answer to this crossword clue is the 6 letter word UNDOCK
I've seen this crossword clue in The Times
U
N
D
O
C
K
More clues leading to the result UNDOCK
Buddy explains!

The straight part
We know that "leave port" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case leave port = undock.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 UNDOCK mean?
UNDOCK verb- To remove (a ship) from a dock.
- To remove from a docking station.
- To drag (a user interface element, such as a toolbar) away from its fixed position so that it floats freely.