Joker aboard truck (5)
I believe the answer is:
wagon
'truck' is the definition.
(thesaurus)
'joker aboard' is the wordplay.
'joker' becomes 'wag' (I've seen this before).
'aboard' becomes 'on'.
'wag'+'on'='WAGON'
(Other definitions for wagon that I've seen before include "Wild West trail vehicle?" , "Vehicle (on which abstainers travel?)" , "On this, one keeps dry!" , "Cart; rail truck" , "Four-wheeled goods vehicle" .)