Vehicle characterized by activity, last to go (3)
I believe the answer is:
bus
'vehicle' is the definition.
(bus is a kind of vehicle)
'characterized by activity last to go' is the wordplay.
'characterized by activity' becomes 'busy'.
'last to go' means to remove the last letter (the last letter goes away).
'busy' with its last letter taken away is 'BUS'.
(Other definitions for bus that I've seen before include "Charabanc" , "Mode of public transport" , "Use transport" , "Means of public transport" , "Large passenger vehicle - small business!" .)