Fulfilled by what's inside (7)
I believe the answer is:
content
I believe this clue is a double definition.
'fulfilled' is the first definition.
(thesaurus)
'what's inside' is the second definition.
(inside a book or article)
'by' acts as a link.
(Other definitions for content that I've seen before include "Peacefully happy" , "Elements" , "Quietly happy" , "Gist" , "what's included" .)