Mass publication (6)
I believe the answer is:
volume
I believe this clue is a double definition.
'mass' is the first definition.
(volume is a kind of mass)
'publication' is the second definition.
(volume is a kind of publication)
(Other definitions for volume that I've seen before include "A book or sound dimension" , "Space occupied by an object" , "Amount of three-dimensional space occupied" , "Book; degree of loudness" , "work" .)