Blank page at the front or back of a book (7)
I believe the answer is:
flyleaf
'blank page at the front or back of a book' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for flyleaf that I've seen before include "Blank page at the beginning or end of a book" , "Blank page at the front or back of a book" , "one's seen in a book" , "Page left blank" .)