Second class fabric (4)
I believe the answer is:
silk
'fabric' is the definition.
(silk is a kind of fabric)
'second class' is the wordplay.
'second' becomes 's' (common abbreviation).
'class' becomes 'ilk' (both can mean a type or variety).
's'+'ilk'='SILK'
(Other definitions for silk that I've seen before include "Item of fine legwear" , "it's taken by QC" , "Smooth material" , "Riding hat covering" , "brief" .)