In favour of dull style (6)
I believe the answer is:
format
'style' is the definition.
(I've seen this before)
'in favour of dull' is the wordplay.
'in favour of' 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 "Structure, layout" , "Size, style and shape, of books say" , "General arrangement of a publication" , "The way something is arranged or set out" , "Style or arrangement of procedure" .)