Acrylic fiber (5)
I believe the answer is:
orlon
'acrylic fiber' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for orlon that I've seen before include "Synthetic acrylic fibre" , "Crease-resistant acrylic fabric" , "Synthetic fabric" , "Synthetic fibre/fabric" .)