Finger the back of a book (5)
I believe the answer is:
index
This clue is a double definition.
'finger' is the first definition.
(index is a kind of finger)
'the back of a book' is the second definition.
(I've seen this before)
(Other definitions for index that I've seen before include "Guide, table" , "Look-up list in book" , "Pointing finger" , "A-to-Z listing" , "One of the fingers" .)