The most common and most likely answer to this crossword clue is the 7 letter word JUGGLER
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Buddy explains!

The straight part
We know that "one keeping balls in the air" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one keeping balls in the air = juggler.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 JUGGLER mean?
JUGGLER noun- Agent noun of juggle; one who either literally juggles objects, or figuratively juggles tasks.
- A person who practices juggling.
- A person who performs tricks using sleight of hand, a conjurer, prestidigitator.
- A magician or wizard.