Book details initially lost (4)
I believe the answer is:
acts
'book' is the definition.
(book of the Bible)
'details initially lost' is the wordplay.
'details' becomes 'facts' (I've seen this in another clue).
'initially lost' means to remove the first letter.
'facts' with its initial letter removed is 'ACTS'.
(Other definitions for acts that I've seen before include "No head for details in book" , "Cast in major play sections" , "these for variety show" , "Pretends" , "straight after John" .)