New article popular — such fine material (8)
I believe the answer is:
'material' is the definition.
(nainsook is a kind of material)
'new article popular such fine' is the wordplay.
'new' becomes 'n'.
'article' becomes 'a' (indefinite article).
'popular' becomes 'in'.
'such' becomes 'so'.
'fine' becomes 'ok' ('OK' can be a synonym of 'fine').
(Other definitions for nainsook that I've seen before include "Material" , "Corner location houses first class new section on cotton fabric" , "Fine soft cotton fabric" .)