Power in Spooner to convert inmates here (4,6)

I believe the answer is:

open prison

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'inmates here' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'power in spooner to convert' is the wordplay.
'power' becomes 'p' (symbol in physics).
'to convert' is an anagram indicator (I've seen 'converts' mean this).
'p'+'in'+'spooner'='pinspooner'
'pinspooner' anagrammed gives 'OPEN PRISON'.

Can you help me to learn more?

(Other definitions for open prison that I've seen before include "jail" , "Gaol for mild offenders" .)

I've seen this clue in The Times.

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