Glossy fabric was a substitute (5)
I believe the answer is:
satin
I believe this clue is a triple definition!
'glossy' is the first definition.
(synonyms)
'fabric' is the second definition.
(satin is a kind of fabric)
'was a substitute' is the third definition.
(to sit in for someone is to be their substitute)
(Other definitions for satin that I've seen before include "Material with glossy front and dull back" , "Shiny solk" , "Lustrous fabric" , "Stain found in cloth" , "Stain on shiny fabric" .)