Something that protects you from bottom-smack (4,4)
I believe the answer is:
seat belt
'something that protects you' is the definition.
I can't tell whether this defines the answer.
'bottom-smack' is the wordplay.
'bottom' becomes 'seat' (synonyms).
'smack' becomes 'belt' (I've seen this before).
'seat'+'belt'='SEAT-BELT'
'from' is the link.
Can you help me to learn more?
(Other definitions for seat belt that I've seen before include "Protective device to be worn in car" , "Protective strap in car" , "Safety device worn in car" , "Testable (anag)" , "Safety strap worn in car" .)