Cast off ancient book (4)
I believe the answer is:
acts
'ancient book' is the definition.
'acts' can be an answer for 'book' (book of the Bible). I am unsure of the 'ancient' bit.
'cast off' is the wordplay.
'off' indicates anagramming the letters.
'cast' anagrammed gives 'ACTS'.
Can you help me to learn more?
(Other definitions for acts that I've seen before include "the entertainment for a revue?" , "Cast in major divisions of plays, like Shakespeare's" , "Goes" , "Cast in major play sections" , "Follower of John" .)