Posh menu unusually plain (8)
I believe the answer is:
homespun
'plain' is the definition.
(thesaurus)
'posh menu unusually' is the wordplay.
'unusually' indicates anagramming the letters.
'posh'+'menu'='poshmenu'
'poshmenu' anagrammed gives 'HOMESPUN'.
(Other definitions for homespun that I've seen before include "Artless - posh menu (anag)" , "Unsophisticated - not made in mill" , "Simple, unsophisticated" , "Rustic" , "Unsophisticated -- posh menu (anag)" .)