Get cover quickly behind vehicle (8)
I believe the answer is:
carapace
'get cover' is the definition.
I can't judge whether this definition defines the answer.
'quickly behind vehicle' is the wordplay.
'quickly' becomes 'apace' ('apace' can be a synonym of 'quickly').
'behind' 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" , "Shell on back of tortoise or crustacean" , "soft top? Hardly" , "Pace a car around a shell, of a tortoise say" , "Hard shell of some animals" .)