Room to turn and mislay plant material (9)
I believe the answer is:
cellulose
'plant material' is the definition.
(I've seen this before)
'room to turn and mislay' is the wordplay.
'room' becomes 'cell' (cell is a kind of room).
'to turn' becomes 'U' (a 'U-turn' is a kind of turn).
'and' means one lot of letters go next to another.
'mislay' becomes 'lose' (synonyms).
'cell'+'u'+'lose'='CELLULOSE'
(Other definitions for cellulose that I've seen before include "O cull eels for what causes the pitted look" , "Plant material" , "Polysaccharide" , "Substance in wood pulp" , "dietary fibre" .)