Vehicles crossing America in convoy (7)
I believe the answer is:
caravan
'convoy' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
(Other definitions for caravan that I've seen before include "Group of desert voyagers" , "Group travelling together across a desert in single file" , "Camel train" , "Towed home" , "Trailer with living quarters" .)