Layout possibly of a new mart? (6)
I believe the answer is:
format
'layout' is the definition.
(thesaurus)
'possibly of a new mart?' is the wordplay.
'possibly' indicates anagramming the letters.
'a new' indicates an anagram.
'of' with letters rearranged gives 'fo'.
'mart' is an anagram of 'rmat'.
'fo'+'rmat'='FORMAT'
(Other definitions for format that I've seen before include "Broadsheet or tabloid, say" , "Style or arrangement of a TV programme" , "Size, style and shape, of books say" , "Style or arrangement of procedure" , "The shape and size, of a book say" .)