Plant fibre (5)
I believe the answer is:
sisal
'plant fibre' is the definition.
(I know that sisal is a type of plant fibre)
This is all the clue.
(Other definitions for sisal that I've seen before include "Rope fibre from leaves of tropical plant" , "Hemp fibre" , "Natural fibre" , "Agave cultivated for its fibre" , "Agave yielding fibre" .)