Essence of chapter missing from long book (7)
I believe the answer is:
epitome
'essence' is the definition.
(I've seen this before)
'chapter missing from long book' is the wordplay.
'chapter' becomes 'c' (abbreviation).
'missing from' is a deletion indicator.
'long' becomes 'epic' (as in an epic story).
'book' becomes 'tome' (a tome is a book).
'epic' with 'c' taken out is 'epi'.
'epi'+'tome'='EPITOME'
'of' is the link.
(Other definitions for epitome that I've seen before include "Personification" , "Summary; embodiment" , "Something embodying perfectly a stated quality or idea" , "Embodiment, type" , "Perfect example of a quality" .)