Malign fiction including book line (5)
I believe the answer is:
libel
'malign' is the definition.
(I've seen this in another clue)
'fiction including book line' is the wordplay.
'fiction' becomes 'lie' (fiction can mean a lie or fabrication).
'including' indicates putting letters inside.
'book' becomes 'b' (abbreviation for book).
'line' becomes 'l' (used when specifying particular lines from a poem).
'lie' placed around 'b' is 'libe'.
'libe'+'l'='LIBEL'
(Other definitions for libel that I've seen before include "Written defamation" , "Calumnious writing" , "Write ill of" , "Actionable words" , "A published false statement that damages someone" .)