Set to work seriously (3,4,2,8)
I believe the answer is:
get down to business
'set to work seriously' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for get down to business that I've seen before include "Start the real work" , "deliver feathers for trading company" , "Begin in earnest" , "Set to work seriously" , "Start work in earnest" .)