Favouring dull layout (6)
I believe the answer is:
format
'layout' is the definition.
(thesaurus)
'favouring dull' is the wordplay.
'favouring' becomes 'for' (I've seen this before).
'dull' becomes 'mat' ('mat' can be similar in meaning to 'dull').
'for'+'mat'='FORMAT'
(Other definitions for format that I've seen before include "The size and shape of something" , "The shape and size, of a book say" , "Style or arrangement of a TV programme" , "magazine" , "Shape and style as of book" .)