Layout (6)
'layout' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for format that I've seen before include "General arrangement of a publication" , "Way in which something is presented" , "Secure firmly in position" , "Broadsheet or tabloid, say" , "Shape and size as of a book, lay-out" .)
(Other definitions for design that I've seen before include "Purpose, pattern" , "Pattern - plan" , "Develop or prepare a plan" , "Devise, create, make out a plan" , "Devise, dreate, make out a plan" .)