Returned books are an inspiration (5)
I believe the answer is:
erato
'inspiration' is the definition.
(one of the muses in Greek mythology)
'returned books are' is the wordplay.
'returned' shows that the letters should be reversed in order.
'books' becomes 'OT' (abbreviation for Old Testament).
'ot'+'are'='otare'
'otare' in reverse letter order is 'ERATO'.
'an' is the link.
(Other definitions for erato that I've seen before include "Greek muse - orate (anag)" , "One of nine" , "inspiration for poetry" , "Lyric poetry's muse" , "daughter of Zeus" .)