More than one prison is a pit internee gathered again (14)

I believe the answer is:

penitentiaries

'more than one prison' is the definition.
(I know this)

'is a pit internee gathered again' is the wordplay.
'gathered again' indicates an anagram (letters gathered in a new order).
'is'+'a'+'pit'+'internee'='isapitinternee'
'isapitinternee' is an anagram of 'PENITENTIARIES'.

I've seen this clue in The Times.

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