Man-made material (5)
'man-made material' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nylon that I've seen before include "Man-made material" , "may be damaged by running" , "Fibrous material" , "Cover for leg" , "Artificial material" .)
(Another definition for fibreglass that I've seen is " Material used as insulation or to reinforce plastics".)