Fibre used in chairs (4)
I believe the answer is:
hair
'fibre' is the definition.
(I know that hair is a type of protein filament)
'in chairs' is the wordplay.
'in' indicates a hidden word.
'HAIR' is hidden in the letters of 'chairs'.
'used' acts as a link.
(Other definitions for hair that I've seen before include "Showy garden flower" , "Famous hirsute musical with song ` `Aquarius' ' (4)" , "It grows from the skin" , "Red face powder" , "It helps prevent heat loss through the head" .)