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