Idiosyncrasy of returning book unopened (6)
I believe the answer is:
foible
'idiosyncrasy' is the definition.
(synonyms)
'of returning book unopened' is the wordplay.
'returning' says the letters should be written in reverse.
'book' becomes 'bible' (bible is a kind of book).
'unopened' means to remove the first letter.
'of' reversed gives 'fo'.
'bible' with its initial letter taken away is 'ible'.
'fo'+'ible'='FOIBLE'
(Other definitions for foible that I've seen before include "Quirk" , "weakness" , "Harmless peculiarity or mannerism" , "'Mannerism, idiosyncrasy (6)'" , "Harmless peculiarity in person's character" .)