Synthetic fiber (5)
'synthetic fiber' is the definition.
(I know that nylon is a type of synthetic fiber)
This is all the clue.
(Other definitions for nylon that I've seen before include "The first synthetic fibre" , "Fabric (originally from New York and London)" , "Man-made material" , "Artificial fibre" , "Polyamide" .)
(Other definitions for rayon that I've seen before include "Or any (anag) -- fibre" , "Viscose fabric" , "Synthetic fibre" , "Man-made fibre" , "Cellulose textile fibre" .)