Creep rewrote book in every hundred (7)

I believe the answer is:

percent

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'every hundred' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'creep rewrote book' is the wordplay.
'rewrote' is an anagram indicator (I've seen 'rewritten' mean this).
'book' becomes 'NT' (New Testament).
'creep' is an anagram of 'perce'.
'perce'+'nt'='PERCENT'

'in' acts as a link.

Can you help me to learn more?

I've seen this clue in The Mirror.

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