Vehicles, vehicles: about time! (5)
I believe the answer is:
carts
'vehicles' is the definition.
(cart is a kind of vehicle)
'vehicles about time' is the wordplay.
'vehicles' becomes 'cars' (car is a kind of vehicle).
'about' is an insertion indicator.
'time' becomes 't' (abbreviation).
'cars' placed around 't' is 'CARTS'.
(Other definitions for carts that I've seen before include "Wheeled vehicles that can be pushed or pulled" , "Two-wheeled drawn vehicles" .)