New Testament book details originally discounted (4)

I believe the answer is:

acts

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'new testament book' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for acts that I've seen before include "Pretends" , "Performs on stage" , "Fifth NT book" , "New Testament book immediately following the Gospels" , "Deeds; book of the Bible" .)

I've seen this clue in The Telegraph.

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