Fabric, unknown line no Parisian's stocking (5)
I believe the answer is:
nylon
'fabric' is the definition.
(nylon is a kind of fabric)
'unknown line no parisian's stocking' is the wordplay.
'unknown' becomes 'y' (algebra).
'line' becomes 'l'.
'no parisian' becomes 'non' ('no' in French).
'stocking' means one lot of letters goes inside another ('stock' can mean 'carry' or 'contain').
'y'+'l'='yl'
'yl' put within 'non' is 'NYLON'.
(Other definitions for nylon that I've seen before include "Tough synthetic material" , "Hosiery material" , "Plastic used for stockings" , "The first synthetic fibre" , "Artificial yarn/fabric" .)