Section of book (7)
I believe the answer is:
chapter
'section of book' is the definition.
(a book may be split into chapters)
This is all the clue.
(Other definitions for chapter that I've seen before include "Part of book" , "Distinctive period of history" , "religious assembly" , "Sequence - section of book - church body" , "Ecclesiastical assembly or subdivision of a written work" .)