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

The straight part
We know that "Balloon" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case balloon = aerostat.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 AEROSTAT mean?
AEROSTAT noun- An aircraft, such as a dirigible or balloon, that derives its lift from buoyancy rather than from wings or rotors.
- A moored balloon flown in a semi-permanent manner, such as a border patrol monitoring balloon affixed at 18,000 feet (~6 km).