Second chapter, knocked, being discarded (8)
I believe the answer is:
scrapped
'being discarded' is the definition.
(I've seen this in another clue)
'second chapter knocked' is the wordplay.
'second' becomes 's' (common abbreviation).
'chapter' becomes 'c' (abbreviation).
'knocked' becomes 'rapped' ('rap' can be a synonym of 'knock').
's'+'c'+'rapped'='SCRAPPED'
(Other definitions for scrapped that I've seen before include "Abandoned - had a fight" , "Disposed of something useless" , "Disposed of, article or idea, say" , "Junked or discarded" , "abandoned in a heap?" .)