Part of body area includes unknown thin material (7)
I believe the answer is:
organza
'thin material' is the definition.
(I've seen this before)
'part of body area includes unknown' is the wordplay.
'part of body' becomes 'organ'.
'area' becomes 'a' (maths abbreviation).
'includes' indicates putting letters inside.
'unknown' becomes 'z' (algebra).
'organ'+'a'='organa'
'organa' enclosing 'z' is 'ORGANZA'.
(Other definitions for organza that I've seen before include "Thin stiff dress fabric" , "Transparent silky material" , "Transparent silk material" , "Silk (or silk-like) fabric" , "Thin material" .)