Active and energetic (5)
'active and energetic' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for on the go that I've seen before include "Very busy, in constant motion" , "Ever busy" , "Active and energetic" , "Active, always doing something" , "Keeping busy, not stopping" .)
(Other definitions for brisk that I've seen before include "Quick, active, energetic" , "Vigorous" , "Lively, invigorating" , "Quick and invigorating" , "Bustling" .)