Interval almost over for book size (6)
I believe the answer is:
octavo
'book size' is the definition.
(I've seen this before)
'interval almost over' is the wordplay.
'interval' becomes 'octave' (musical interval).
'almost' means to remove the last letter (most of the word but not all of it).
'over' becomes 'o' (cricket abbreviation).
'octave' with its last letter taken off is 'octav'.
'octav'+'o'='OCTAVO'
'for' is the link.
(Other definitions for octavo that I've seen before include "Common book size" , "Size of a book, pages are made by folding a sheet of paper three times to form eight leaves (6)" , "An eight-leaved book or page" , "Size of book page" .)