The most common and most likely answer to this crossword clue is the 8 letter word FORESTAY
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More clues leading to the result FORESTAY
Buddy explains!

The straight part
We know that "Nautical brace" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nautical brace = forestay.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 FORESTAY mean?
FORESTAY noun- A stay that extends from the top of the foremast to the bow or bowsprit of a sailing ship
- To stay beforehand; secure or fasten with or as with a forestay.
- To stay, delay, postpone, or hinder beforehand; forestall; prevent.