Briefly crowded, public transport vehicle (3)
I believe the answer is:
bus
'public transport vehicle' is the definition.
(buses are a type of public transport)
'briefly crowded' is the wordplay.
'briefly' means to remove the last letter.
'crowded' becomes 'busy' (I've seen this in another clue).
'busy' with its last letter taken off is 'BUS'.
(Other definitions for bus that I've seen before include "Road vehicle able to carry many passengers" , "Means of public transport" , "Of projectiles; (go) mad (slang)" , "Mode of public transport" , "Something caught" .)