Shell drivers returning quickly (8)
I believe the answer is:
carapace
'shell' is the definition.
(shell of an animal)
'drivers returning quickly' is the wordplay.
'drivers' becomes 'rac' (Royal Automobile Club).
'returning' is a reversal indicator.
'quickly' becomes 'apace' (synonyms).
'rac' in reverse letter order is 'car'.
'car'+'apace'='CARAPACE'
(Other definitions for carapace that I've seen before include "Shell on back of tortoise or crustacean" , "Hard shell of tortoise, crab etc" , "hardtop" , "Protective cover" , "Hard thick shell, of a turtle say" .)