Prosaic and dull, cart off roughly (6,2,4)
I believe the answer is:
matter of fact
'prosaic' is the definition.
('matter-of-fact' can be a synonym of 'prosaic')
'dull cart off roughly' is the wordplay.
'dull' becomes 'matte' ('matte' can be similar in meaning to 'dull'**).
'roughly' indicates anagramming the letters.
'cart'+'off'='cartoff'
'cartoff' is an anagram of 'roffact'.
'matte'+'roffact'='MATTER-OF-FACT'
'and' acts as a link.
(Other definitions for matter of fact that I've seen before include "Unemotional and practical" , "Down to earth" , "Emotionless" , "reality" , "Cold" .)