Commonplace affair, rotten deed (6-2-4)
I believe the answer is:
matter-of-fact
'commonplace' is the definition.
(thesaurus)
'affair rotten deed' is the wordplay.
'affair' becomes 'matter' ('matter' can be a synonym of 'affair'**).
'rotten' becomes 'off' (I've seen this before).
'deed' 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 "Down to earth" , "Commonplace" , "Straightforward and unemotional" , "Unemotional and practical" , "Unimaginative, emotionless" .)