The most common and most likely answer to this crossword clue is the 8 letter word DISTENDS
I've seen this crossword clue in The Guardian
D
I
S
T
E
N
D
S
Buddy explains!

The straight part
We know that "balloons" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case balloons = distends.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 DISTENDS mean?
DISTENDS verb- To extend or expand, as from internal pressure; to swell
- To extend; to stretch out; to spread out.
- To cause to swell.
- To cause gravidity.