Comedian aboard cart (5)
I believe the answer is:
wagon
'cart' is the definition.
(I've seen this before)
'comedian aboard' is the wordplay.
'comedian' becomes 'wag' (I've seen this before).
'aboard' becomes 'on'.
'wag'+'on'='WAGON'
(Other definitions for wagon that I've seen before include "Cart; rail truck" , "Four-wheeled goods vehicle" , "Wheeled vehicle drawn by a horse" , "Freight truck" , "Horse-drawn vehicle" .)