Coconut husk fiber (4)
I believe the answer is:
'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" .)