Continue to move to and fro, as a vehicle (5)
I believe the answer is:
wagon
'a vehicle' is the definition.
(I know that wagon is a type of wheeled vehicle)
'continue to move to and fro' is the wordplay.
'continue' becomes 'on' (I've seen this in other clues).
'to' means one lot of letters go next to another (I've seen this in other clues).
'move to and fro' becomes 'wag' (I've seen this in another clue).
'on' put after 'wag' is 'WAGON'.
'as' is the link.
(Other definitions for wagon that I've seen before include "On this, one keeps dry!" , "Trailer, railway truck" , "Wild West trail vehicle?" , "Freight truck" , "Vehicle carrying goods" .)