Natural fibre (4)
'natural fibre' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cotton that I've seen before include "Golfing man" , "Source of cloth" , "This wool is a fluffy wadding" , "Cloth woven from a plant" , "Cloth made from fine hairs of seed pod" .)
(Other definitions for wool that I've seen before include "What can be pulled over eyes to deceive" , "Sheep's fleece" , "Hair of sheep etc" , "Sheep's coat" , "Sheep's hair" .)