The most common and most likely answer to this crossword clue is the 6 letter word STEEVE
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The straight part
We know that "pack ship tightly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pack ship tightly = steeve.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 STEEVE mean?
STEEVE noun- The angle which a bowsprit makes with the horizon, or with the line of the vessel's keel.
- A spar, with a block at one end, used in stowing cotton bales and similar cargo needing to be packed tightly.
- To project upward, or make an angle with the horizon or with the line of a vessel's keel; said of the bowsprit, etc.
- To stow, as bales in a vessel's hold, by means of a steeve.