The most common and most likely answer to this crossword clue is the 10 letter word BATTLESHIP
I've seen this crossword clue in The Guardian
B
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More clues leading to the result BATTLESHIP
Buddy explains!

The straight part
We know that "man-of-war" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case man-of-war = battleship.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 BATTLESHIP mean?
BATTLESHIP noun- Large capital warship displacing tens of thousands of tons, heavily armoured and armed with big guns. Battleships are now obsolescent, replaced by smaller vessels with guided missiles. Types: dreadnought, pre-dreadnought.
- Non-functional rocket stage, used for configuration and integration tests.
- A guessing game played on grid paper, see Battleship (game)