Shell swiftly locked on vehicle (8)
I believe the answer is:
carapace
'shell' is the definition.
('carapace' can be a synonym of 'shell')
'swiftly locked on vehicle' is the wordplay.
'swiftly' becomes 'apace' ('apace' can be a synonym of 'quickly').
'locked on' is a charade indicator (letters next to each other).
'vehicle' becomes 'car' (car is a kind of vehicle).
'apace' put after 'car' is 'CARAPACE'.
(Other definitions for carapace that I've seen before include "Upper shell of tortoise" , "Hard upper shell" , "Hard thick shell, of a turtle say" , "Structure on back of tortoise or turtle" , "Shell on back of tortoise or crustacean" .)