The most common and most likely answer to this crossword clue is the 9 letter word AUDIENCES
I've seen this crossword clue in The Guardian
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More clues leading to the result AUDIENCES
Buddy explains!

The straight part
We know that "groups of spectators" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case groups of spectators = audiences.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 AUDIENCES mean?
AUDIENCES noun- A group of people within hearing; specifically, a large gathering of people listening to or watching a performance, speech, etc.
- Hearing; the condition or state of hearing or listening.
- A widespread or nationwide viewing or listening public, as of a TV or radio network or program.
- A formal meeting with a state or religious dignitary.
- The readership of a book or other written publication.
- A following.
- An audiencia (judicial court of the Spanish empire), or the territory administered by it.