Paper mill material (4)
I believe the answer is:
pulp
'paper mill material' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pulp that I've seen before include "Soft fleshy part of a fruit" , "Soft, soggy mass" , "Cheap fiction" , "Mush" , "Soft plant tissue" .)