Prosaic concern, not on law (6-2-4)
I believe the answer is:
matter-of-fact
'prosaic' is the definition.
(synonyms)
'concern not on law' is the wordplay.
'concern' becomes 'matter' (matter is a kind of concern).
'not on' becomes 'off' ('off' is the opposite to 'on').
'law' becomes 'act' (I've seen this before).
'matter'+'off'+'act'='MATTER-OF-FACT'
(Other definitions for matter-of-fact that I've seen before include "Unimaginative, emotionless" , "Commonplace" , "Down to earth" , "Straightforward and unemotional" , "Unemotional and practical" .)