Returned books are an inspiration (5)
I believe the answer is:
erato
'an inspiration' is the definition.
(thesaurus)
'returned books are' is the wordplay.
'returned' says the letters should be written backwards.
'books' becomes 'ot' (Old Testament).
'ot'+'are'='otare'
'otare' in reverse letter order is 'ERATO'.
(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" .)