Ernestine freed people who are locked up (9)

I believe the answer is:

internees

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'people who are locked up' is the definition.
The answer and definition are not the same part of speech.

'ernestine freed' is the wordplay.
'freed' is an anagram indicator.
'ernestine' anagrammed gives 'INTERNEES'.

Can you help me to learn more?

(Another definition for internees that I've seen is " POWs".)

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