The most common and most likely answer to this crossword clue is the 9 letter word STREAMING
I've seen this crossword clue in Financial Times
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Buddy explains!
The straight part
We know that "moving in large numbers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case moving in large numbers = streaming.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 STREAMING mean?
STREAMING verb- To flow in a continuous or steady manner, like a liquid.
- To extend; to stretch out with a wavy motion; to float in the wind.
- To discharge in a stream.
- To push continuous data (e.g. music) from a server to a client computer while it is being used (played) on the client.
- Flowing or moving in continuous succession, like fluid in a stream.
- Movement as a stream.
- The transmission of digital audio or video, or the reception or playback of such data without first storing it.
- Division of classes into academic streams.
- The working of alluvial deposits to get ore.