Start up (4)
'start up' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for boot that I've seen before include "Profit" , "Sturdy shoe; start (a computer)" , "Clothing; kick" , "may be packed for a journey" , "Dismiss" .)
(Other definitions for activate that I've seen before include "Switch on" , "Cause to start, set in motion" , "Launch" , "Galvanize" , "Start working" .)