Saint shut up as church wasted power (5,5)

I believe the answer is:

spent force

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'wasted power' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'saint shut up as church' is the wordplay.
'saint' becomes 's'.
'shut up' becomes 'pen' (pen can mean to shut in or enclose).
'as' becomes 'tfor' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'church' becomes 'ce' (Church of England).
's'+'pen'+'tfor'+'ce'='SPENT FORCE'

Can you help me to learn more?

(Other definitions for spent force that I've seen before include "power exhausted" , "Has-been" .)

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