The most common and most likely answer to this crossword clue is the 9 letter word MESSMATES
M
E
S
S
M
A
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The straight part
We know that "men on board" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case men on board = messmates.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 MESSMATES mean?
MESSMATES noun- An associate with whom one shares a mess (eating place) on a ship
- Any of various eucalypts with rough bark, specifically Eucalyptus obliqua.