To be in earnest (4,8)
I believe the answer is:
'to be in earnest' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mean business that I've seen before include "underhand affair" , "Intend to take positive action" , "Be deadly serious" , "Sees man in bus (anag) - be in earnest" , "usury" .)