Well under way (2,4,5)
I believe the answer is:
in full swing
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">well under way</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for in full swing that I've seen before include "going from one side to the other the whole way" , "Well underway" , "well under way" , "At the height of activity" .)