Gap, missing portion (6)
I believe the answer is:
lacuna
I believe this is a double definition.
'gap' is the first definition.
(I know that lacuna is a type of gap)
'missing portion' is the second definition.
(I've seen this before)
(Other definitions for lacuna that I've seen before include "break" , "Blank gap or missing part" , "Space in book or manuscript" , "A gap, maybe in a book" , "A gap, or section missing from a book" .)