Underway (5)
There are a few possibilities for this:
in motion
'underway' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for begun that I've seen before include "(Has) started" , "Commenced" , "Opened" .)
'underway' is the definition.
(synonyms)
This is all the clue.
(Other definitions for afoot that I've seen before include "Going on" , "Brewing" , "Stirring" , "Currently in progress" , "Making progress" .)
(Another definition for in motion that I've seen is " Not at rest, moving".)