Hard top that could make vehicle fast (8)
I believe the answer is:
carapace
'hard top' is the definition.
(a carapace is a shell)
'vehicle fast' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'fast' becomes 'apace' (synonyms).
'car'+'apace'='CARAPACE'
'that could make' acts as a link.
(Other definitions for carapace that I've seen before include "Hard shell of some animals" , "Hard shell of tortoise, crab etc" , "Shell on back of tortoise or crustacean" , "soft top? Hardly" , "Firm covering" .)