A natural fibre (6)
I believe the answer is:
cotton
'a natural fibre' is the definition.
(I know that cotton is a type of plant fiber)
This is the entire clue.
(Other definitions for cotton that I've seen before include "Source of cloth" , "This wool is a fluffy wadding" , "Substance like fine wool" , "Soft fabric and fluffy wool (6" , "Fabric made from tropical plant - old man river don't plant it" .)