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 know nothing about this answer so I cannot tell whether this works.

'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 can't explain this - if you can you should believe this answer much more).
'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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