Vehicle is endless worry (3)
I believe the answer is:
car
'vehicle' is the definition.
(car is a kind of vehicle)
'endless worry' is the wordplay.
'endless' means to remove the last letter.
'worry' becomes 'care' (eg without a care means without a worry).
'care' with its last letter taken away is 'CAR'.
'is' acts as a link.
(Other definitions for car that I've seen before include "Conveyance" , "Wheeled vehicle" , "Passenger vehicle" , "Automobile" , "Road vehicle; rail coach" .)