Vehicles honked, first travelling to the rear (5)
I believe the answer is:
tanks
'vehicles' is the definition.
(tank is a kind of vehicle)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for tanks that I've seen before include "Bombs" , "Armed vehicles" , "Armored military vehicles" , "Armoured military vehicles" , "Reservoirs - military vehicles" .)