The vehicle a joker boarded? (5)
I believe the answer is:
wagon
'the vehicle' is the definition.
(wagon is a kind of vehicle)
'a joker boarded?' is the wordplay.
'a joker' becomes 'wag' (I've seen this in another clue).
'boarded?' becomes 'on' (I can't explain this - if you can you should give a lot more credence to this answer).
'wag'+'on'='WAGON'
Can you help me to learn more?
(Other definitions for wagon that I've seen before include "Vehicle for transporting goods" , "Wild West trail vehicle?" , "Freight truck" , "Four-wheeled goods vehicle" , "On this, one keeps dry!" .)