The vehicle a joker gets on (5)
I believe the answer is:
wagon
'the vehicle' is the definition.
(wagon is a kind of vehicle)
'a joker gets on' is the wordplay.
'a joker' becomes 'wag' (I've seen this in another clue).
'gets' says to put letters next to each other.
'wag'+'on'='WAGON'
(Other definitions for wagon that I've seen before include "Van, lorry" , "Vehicle for transporting goods" , "Trailer, railway truck" , "Four-wheeled goods vehicle" , "Railway freight vehicle" .)