Humorist aboard a vehicle (5)

I believe the answer is:

wagon

'a vehicle' is the definition.
(wagon is a kind of vehicle)

'humorist aboard' is the wordplay.
'humorist' becomes 'wag' (wag is a kind of humorist).
'aboard' becomes 'on'.
'wag'+'on'='WAGON'

(Other definitions for wagon that I've seen before include "Vehicle carrying goods" , "Wheeled vehicle drawn by a horse" , "Vehicle (on which abstainers travel?)" , "Vehicle for transporting goods" , "Trailer, railway truck" .)

I've seen this clue in the Evening Standard.

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