Quickly going after vehicle, hardtop? (8)
I believe the answer is:
carapace
'hardtop?' is the definition.
(I've seen this before)
'quickly going after vehicle' is the wordplay.
'quickly' becomes 'apace' ('apace' can be a synonym of 'quickly').
'going after' is a charade indicator (letters next to each other).
'vehicle' becomes 'car' (car is a kind of vehicle).
'apace' after 'car' is 'CARAPACE'.
(Other definitions for carapace that I've seen before include "hardtop" , "Hard outer covering of some animals" , "Thick outer shell of tortoises, crabs etc" , "soft top? Hardly" , "Hard shell of tortoise, crab etc" .)