Prisoner is useful and is not wasteful (9)

I believe the answer is:

conserves

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'prisoner is useful and' is the definition.
I can't judge whether this definition defines the answer.

'not wasteful' is the wordplay.
'not' becomes 'con' (I've seen this in other clues).
'wasteful' becomes 'serves' (I can't explain this - if you can you should believe this answer much more).
'con'+'serves'='CONSERVES'

'is' acts as a link.

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