Set to work seriously (3,4,2,8)
I believe the answer is:
get down to business
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">set to work seriously</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)