US writer's pen found under Old Testament book (4,5)

'us writer's' is the definition.
(Ezra Pound was a US poet)

'pen found under old testament book' is the wordplay.
'pen' becomes 'pound' (both can mean an animal enclosure).
'found under' means one lot of letters go next to another.
'old testament book' becomes 'ezra' (Ezra is a book of the Old Testament).
'pound' put after 'ezra' is 'EZRA POUND'.