Vehicle occupied, briefly (3)
I believe the answer is:
bus
'vehicle' is the definition.
(bus is a kind of vehicle)
'occupied briefly' is the wordplay.
'occupied' becomes 'busy' (similar in meaning).
'briefly' means to remove the last letter.
'busy' with its final letter taken off is 'BUS'.
(Other definitions for bus that I've seen before include "Public service vehicle" , "Coach" , "Of projectiles; (go) mad (slang)" , "Use transport" , "Large passenger vehicle - small business!" .)