Large vehicle (5)
(Other definitions for van that I've seen before include "Commercial vehicle at head of army" , "Forefront as of an army" , "Commercial road vehicle" , "camper?" , "Commercial vehicle in the lead?" .)
'large vehicle' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bus that I've seen before include "Public service vehicle" , "Public transport vehicle" , "eg Double-decker" , "Use transport" , "Mode of public transport" .)
(Other definitions for truck that I've seen before include "Bargain" , "See 14" , "Exchange" , "Goods vehicle" , "Large road vehicle" .)