The most common and most likely answer to this crossword clue is the 7 letter word SEAWARD
I've seen this crossword clue in The Guardian
S
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A
W
A
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More clues leading to the result SEAWARD
- Heading for the main prize after serious effort initially
- Novel was read as boat goes out of port
- A wader’s swimming downstream
- Kent gets a regulatory area facing offshore
- Like sailor going out from Kent, say, with prize
- Society event’s first prize has a marine destination
- Died after the main battle, away from shore
- Reportedly look at child going in the direction of the main
Buddy explains!

The straight part
We know that "as boat goes out of port" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as boat goes out of port = seaward.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 SEAWARD mean?
SEAWARD adjective- Being in or facing towards the sea, as opposed to the land.
- In the direction of the sea, toward the sea.