Shell makes vehicle fast! (8)
I believe the answer is:
carapace
'shell' is the definition.
('carapace' can be a synonym of 'shell')
'vehicle fast' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'fast' becomes 'apace' ('apace' can be a synonym of 'fast').
'car'+'apace'='CARAPACE'
'makes' acts as a link.
(Other definitions for carapace that I've seen before include "Protective cover" , "Upper shell of tortoise" , "Shell on back of tortoise or crustacean" , "Hard thick shell, of a turtle say" , "Pace a car around a shell, of a tortoise say" .)