Crossword Genius

Imprisons one by one, say (7)

Ross

I believe the answer is:

interns

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'imprisons' is the definition.
The definition and answer can be both to do with social activities as well as being verbs in their -s form.
Perhaps you can see a link between them that I don't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for interns that I've seen before include "those confined" , "Hospital residents" .)

I've seen this clue in The Daily Express.
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