Beetle has one motor at full speed (8)
I believe the answer is:
carapace
'beetle has one' is the definition.
(carapace is a type of shell)
'motor at full speed' is the wordplay.
'motor' becomes 'car' (informal term for car).
'at full speed' becomes 'apace' (I've seen this in another clue).
'car'+'apace'='CARAPACE'
(Other definitions for carapace that I've seen before include "Crab's thick protective shield" , "Structure on back of tortoise or turtle" , "Hard thick shell of some creatures" , "Hard thick shell, of a turtle say" , "Hard shell of tortoise, crab etc" .)