Perform decisively in shabby affair (4,8)
I believe the answer is:
mean business
'perform decisively' is the definition.
(a decisive person is said to mean business)
'shabby affair' is the wordplay.
'shabby' becomes 'mean' (both can mean unkind or unfair).
'affair' becomes 'business'.
'mean'+'business'='MEAN BUSINESS'
'in' acts as a link.
(Other definitions for mean business that I've seen before include "usury" , "Sees man in bus (anag) - be in earnest" , "Be deadly serious" , "Intend to take positive action" , "underhand affair" .)