Polysaccharide; the main constituent of plant cell walls (9)
I believe the answer is:
cellulose
I believe this is a double definition.
'polysaccharide' is the first definition.
(I know that cellulose is a type of polysaccharide)
'the main constituent of plant cell walls' is the second definition.
(I've seen this before)
(Other definitions for cellulose that I've seen before include "Polysaccharide" , "dietary fibre" , "film-makers use it" , "Substance in plants -- so cull eel (anag)" , "O cull eels for what causes the pitted look" .)