Vehicle requiring constant skill (4)
I believe the answer is:
cart
'vehicle' is the definition.
(cart is a kind of vehicle)
'constant skill' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'skill' becomes 'art' (I've seen this before).
'c'+'art'='CART'
'requiring' is the link.
(Other definitions for cart that I've seen before include "Carrying vehicle" , "Two-wheeled, drawn vehicle" , "Hump" , "Pulled vehicle" , "Old two-wheeled goods vehicle" .)