Second subject to split in books (6)
I believe the answer is:
sunder
'split in books' is the definition.
(literary term for 'split')
'second subject' is the wordplay.
'second' becomes 's' (common abbreviation).
'subject' becomes 'under' (I've seen this in other clues).
's'+'under'='SUNDER'
'to' acts as a link.
(Other definitions for sunder that I've seen before include "Nursed so as to tear apart" , "Break in pieces" , "Tear apart, sever" , "Break or tear apart, maybe a relationship" , "Split apart" .)