Shell the vehicle quickly (8)
I believe the answer is:
carapace
'shell' is the definition.
(shell of an animal)
'vehicle quickly' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'quickly' becomes 'apace' (synonyms).
'car'+'apace'='CARAPACE'
'the' acts as a link.
(Other definitions for carapace that I've seen before include "Tortoise's upper shell" , "Hard thick shell, of a turtle say" , "Pace a car around a shell, of a tortoise say" , "Hard upper shell" , "Protective cover" .)