Part of a book gets Enid upset and cross (5)
I believe the answer is:
index
'part of a book' is the definition.
(I know that front and back matter of a book has part index)
'enid upset and cross' is the wordplay.
'upset' is an anagram indicator.
'and' says to put letters next to each other.
'cross' becomes 'x' (X is cross-shaped).
'enid' is an anagram of 'inde'.
'inde'+'x'='INDEX'
'gets' acts as a link.
(Other definitions for index that I've seen before include "Register of contents" , "Sign of something" , "Concordance" , "Alphabetical register" , "A-to-Z listing" .)