Big truck (3)
'big truck' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rig that I've seen before include "Apparatus; (boat's) sails" , "Oil platform" , "engineer or > doctor" , "Arrangement of masts, spars and sails on a ship" , "Apparatus; outfit" .)
(Other definitions for van that I've seen before include "Leading part" , "trailer" , "Leading point of army" , "Leading military units" , "Forefront, vehicle" .)