Index in the story book (5)
I believe the answer is:
table
'index' is the definition.
(thesaurus)
'in the story book' is the wordplay.
'in the' means one lot of letters goes inside another.
'story' becomes 'tale' ('tale' can be a synonym of 'story').
'book' becomes 'b'.
'tale' enclosing 'b' is 'TABLE'.
(Other definitions for table that I've seen before include "Array of figures" , "Statistical chart" , "Move; furniture" , "Board off which dinner's eaten" , "Horizontal furniture" .)