Blank page at the front or back of a book (7)
I believe the answer is:
flyleaf
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">blank page at the front or back of a book</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)