Vehicle a worry, mostly (3)
I believe the answer is:
car
'vehicle' is the definition.
(car is a kind of vehicle)
'a worry mostly' is the wordplay.
'a worry' becomes 'care' (I've seen this before).
'mostly' means to remove the last letter.
'care' with its last letter taken away is 'CAR'.
(Other definitions for car that I've seen before include "Motor vehicle" , "Passenger vehicle" , "Vehicle carrying vehicles" , "Automobile" , "Means of transport" .)