Plant material (9)
I believe the answer is:
cellulose
'plant material' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cellulose that I've seen before include "film-makers use it" , "Polysaccharide" , "Plant material" , "Main constituent of plant cell walls" , "O cull eels for what causes the pitted look" .)