Shield quickly attached to vehicle (8)
I believe the answer is:
carapace
'shield' is the definition.
(thesaurus)
'quickly attached to vehicle' is the wordplay.
'quickly' becomes 'apace' ('apace' can be a synonym of 'quickly').
'attached to' 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 "Pace a car around a shell, of a tortoise say" , "Crab's thick protective shield" , "hardtop" , "Hard outer covering of some animals" , "Hard shell of some animals" .)