It’s an alternative material (5)
I believe the answer is:
satin
'material' is the definition.
(I know that satin is a type of material)
'it's an alternative' is the wordplay.
'alternative' indicates anagramming the letters (an alternative ordering of the letters).
'its'+'an'='itsan'
'itsan' with letters rearranged gives 'SATIN'.
(Other definitions for satin that I've seen before include "Silky material" , "covered with?" , "Shiny solk" , "Stain on silky cloth" , "Stain found in cloth" .)