Coconut husk fibre (4)
I believe the answer is:
coir
'coconut husk fibre' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for coir that I've seen before include "Coconut husk fibre used for making matting" , "growing medium" , "Fibre from coconut husks" , "Coarse fibre from a coconut husk" .)