Opening section of book cut up (3)
I believe the answer is:
gap
'opening' is the definition.
(gap is a kind of opening)
'section of book cut up' is the wordplay.
'section of book' becomes 'page'.
'cut' means to remove the last letter.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'page' with its last letter removed is 'pag'.
'pag' reversed gives 'GAP'.
(Other definitions for gap that I've seen before include "Room" , "A break or opening as in a hedge" , "Disparity" , "Interval, break" , "Vacancy" .)