The most common and most likely answer to this crossword clue is the 8 letter word CARAPACE
I've seen this crossword clue in The Guardian
C
A
R
A
P
A
C
E
More clues leading to the result CARAPACE
- Motor at speed in soft top? Hardly
- Shell makes vehicle fast
- Shield quickly attached to vehicle
- Shell of vehicle quickly assembled
- Charlie, a star popular musician, is a tough case
- Get cover quickly behind vehicle
- Case of vehicle speeding
- Walk behind vehicle with a protective covering
- Vehicle rapidly becoming a shell
- Vehicle takes a step to become a hardtop
Buddy explains!

The straight part
We know that "a shell" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a shell = carapace.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CARAPACE mean?
CARAPACE noun- A hard protective covering of bone or chitin, especially one which covers the dorsal portion of an animal.
- In figurative use